Nonthermal plasma synthesis has emerged as a viable alternative to nanocrystal synthesis in the liquid phase or by other gas phase based methods. The nonequilibrium environment containing free charge ...carriers enables the synthesis of nanocrystals with excellent crystallinity and narrow size distributions. This paper reviews the fundamental mechanisms involved in the synthesis of nanocrystals with nonthermal plasmas. It discusses the luminescent properties of plasma-produced silicon nanocrystals and their application in devices such as light emitting diodes. The ability of plasma synthesis to generate doped nanocrystals is a particularly appealing attribute. We present boron and phosphorous doped silicon nanocrystals and review their applications as near infrared plasmonic materials. Finally, the author presents his view of some important research needs in the area of nonthermal plasma synthesis of nanocrystals.
Silicon quantum dots are attractive materials for luminescent devices and bioimaging applications. For these light-emitting applications, higher photoluminescence efficiency is desired in order to ...achieve better device performance. Nonthermal plasma synthesis successfully allows for the continuous production of silicon nanocrystals, but postprocessing is necessary to improve photoluminescence quantum yields so that nanocrystals can be used for luminescence applications. In this work, we demonstrate an all-aerosol-phase synthesis and processing route that integrates nonthermal plasma synthesis, plasma-assisted surface functionalization with alkene ligands, and in-flight annealing within one flow stream. Here, luminescent silicon nanocrystals are synthesized and postprocessed on a time scale of only 100 ms, which is orders of magnitude faster than previous synthesis and functionalization schemes. The as-produced silicon nanocrystals have photoluminescence quantum yields exceeding 20%, which is a 5-fold increase compared to previous silicon nanocrystals synthesized with all-aerosol-phase approaches. We attribute the enhanced photoluminescence to the reduced “dark” nanocrystal fraction due to reduction of dangling bond density and desorption of surface silyl species induced by the in-flight annealing. We also demonstrate that the ligand coverage plays a minor role for the photoluminescence properties, but that the nature of the silicon hydride surface groups is a major factor.
We demonstrate highly efficient electroluminescence from silicon nanocrystals (SiNCs). In an optimized nanocrystal-organic light-emitting device, peak external quantum efficiencies of up to 8.6% can ...be realized with emission originating solely from the SiNCs. The high efficiencies reported here demonstrate for the first time that with an appropriate choice of device architecture it is possible to achieve highly efficient electroluminescence from nanocrystals of an indirect band gap semiconductor.
Nonthermal plasmas have emerged as a viable synthesis technique for nanocrystal materials. Inherently solvent and ligand-free, nonthermal plasmas offer the ability to synthesize high purity ...nanocrystals of materials that require high synthesis temperatures. The nonequilibrium environment in nonthermal plasmas has a number of attractive attributes: energetic surface reactions selectively heat the nanoparticles to temperatures that can strongly exceed the gas temperature; charging of nanoparticles through plasma electrons reduces or eliminates nanoparticle agglomeration; and the large difference between the chemical potentials of the gaseous growth species and the species bound to the nanoparticle surfaces facilitates nanocrystal doping. This paper reviews the state of the art in nonthermal plasma synthesis of nanocrystals. It discusses the fundamentals of nanocrystal formation in plasmas, reviews practical implementations of plasma reactors, surveys the materials that have been produced with nonthermal plasmas and surface chemistries that have been developed, and provides an overview of applications of plasma-synthesized nanocrystals.
Degenerately doped silicon nanocrystals are appealing plasmonic materials due to silicon’s low cost and low toxicity. While surface plasmonic resonances of boron-doped and phosphorus-doped silicon ...nanocrystals were recently observed, there currently is poor understanding of the effect of surface conditions on their plasmonic behavior. Here, we demonstrate that phosphorus-doped silicon nanocrystals exhibit a plasmon resonance immediately after their synthesis but may lose their plasmonic response with oxidation. In contrast, boron-doped nanocrystals initially do not exhibit plasmonic response but become plasmonically active through postsynthesis oxidation or annealing. We interpret these results in terms of substitutional doping being the dominant doping mechanism for phosphorus-doped silicon nanocrystals, with oxidation-induced defects trapping free electrons. The behavior of boron-doped silicon nanocrystals is more consistent with a strong contribution of surface doping. Importantly, boron-doped silicon nanocrystals exhibit air-stable plasmonic behavior over periods of more than a year.
Localized surface plasmon resonances (LSPRs) enable tailoring of the optical response of nanomaterials through their free carrier concentration, morphology, and dielectric environment. Recent efforts ...to expand the spectral range of usable LSPR frequencies into the infrared successfully demonstrated LSPRs in doped semiconductor nanocrystals. Despite silicon’s importance for electronic and photonic applications, no LSPRs have been reported for doped silicon nanocrystals. Here we demonstrate doped silicon nanocrystals synthesized via a nonthermal plasma technique that exhibits tunable LSPRs in the energy range of 0.07–0.3 eV or mid-infrared wavenumbers of 600–2500 cm–1.
To fully deploy the potential of semiconductor nanocrystal films as low-cost electronic materials, a better understanding of the amount of dopants required to make their conductivity metallic is ...needed. In bulk semiconductors, the critical concentration of electrons at the metal-insulator transition is described by the Mott criterion. Here, we theoretically derive the critical concentration nc for films of heavily doped nanocrystals devoid of ligands at their surface and in direct contact with each other. In the accompanying experiments, we investigate the conduction mechanism in films of phosphorus-doped, ligand-free silicon nanocrystals. At the largest electron concentration achieved in our samples, which is half the predicted nc, we find that the localization length of hopping electrons is close to three times the nanocrystals diameter, indicating that the film approaches the metal-insulator transition.
CO2 emissions from steel production account for about 8% of the global anthropogenic CO2 emissions and the majority (over 70%) of these emissions occur during the reduction of iron ore to iron. ...Hence, the steel industry is striving to reduce its dependence on carbon-based energy sources and reducing agents, like the coke used in a traditional blast furnace. Approaches such as hydrogen-based direct reduction are being considered since they can drastically reduce the overall CO2 emissions of the steel-making process. Here, we report an electrified process for reducing iron ore particles using atmospheric pressure hydrogen plasma powered by microwave energy. The process has the potential to be entirely carbon-free and overcome common challenges of other hydrogen reduction approaches, including other plasma-based approaches. Relative reduction rates achieved are as high as 15.5% /s, on par or faster than the highest rates reported in the literature operating at lower temperatures and hydrogen concentrations. When compared to thermal reduction under otherwise close to identical conditions, the microwave plasma reduction is three to four times faster, suggesting the importance of plasma generated reactive species like atomic hydrogen. A promising mass scaling is observed, with increasing the mass load 50 times requiring only 7 times longer reaction, which points to a good potential for further scale-up of the technology.
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•Carbon-free steel making can reduce current global CO2 emissions by up to 8%.•Microwave hydrogen plasma is new zero-carbon method to reduce iron ore to iron.•Rapid reduction of iron oxide to iron is possible within seconds.•Plasma method is faster than thermal reduction due to energetic plasma species.
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