İlker Doğan

Ultrahigh throughput plasma processing of free standing silicon nanocrystals with lognormal size distribution

We demonstrate a method for synthesizing free standing silicon nanocrystals in an argon/silane gas mixture by using a remote expanding thermal plasma. Transmission electron microscopy and Raman spectroscopy measurements reveal that the distribution has a bimodal shape consisting of two distinct groups of small and large silicon nanocrystals with sizes in the range 2–10 nm and 50–120 nm, respectively. We also observe that both size distributions are lognormal which is linked with the growth time and transport of nanocrystals in the plasma. Average size control is achieved by tuning the silane flow injected into the vessel. Analyses on morphological features show that nanocrystals are monocrystalline and spherically shaped. These results imply that formation of silicon nanocrystals is based on nucleation, i.e., these large nanocrystals are not the result of coalescence of small nanocrystals. Photoluminescence measurements show that silicon nanocrystals exhibit a broad emission in the visible region peaked at 725 nm. Nanocrystals are produced with ultrahigh throughput of about 100 mg/min and have state of the art properties, such as controlled size distribution, easy handling, and room temperature visible photoluminescence.

Direct characterization of nanocrystal size distribution using Raman spectroscopy

We report a rigorous analytical approach based on one-particle phonon confinement model to realize direct detection of nanocrystal size distribution and volume fraction by using Raman spectroscopy. For the analysis, we first project the analytical confinement model onto a generic distribution function, and then use this as a fitting function to extract the required parameters from the Raman spectra, i.e., mean size and skewness, to plot the nanocrystal size distribution. Size distributions for silicon nanocrystals are determined by using the analytical confinement model agree well with the one-particle phonon confinement model, and with the results obtained from electron microscopy and photoluminescence spectroscopy. The approach we propose is generally applicable to all nanocrystal systems, which exhibit size-dependent shifts in the Raman spectrum as a result of phonon confinement.

Nucleation of silicon nanocrystals in a remote plasma without subsequent coagulation

We report on the growth mechanism of spherical silicon nanocrystals in a remote expanding Ar plasma using a time-modulated SiH4 gas injection in the microsecond time range. Under identical time-modulation parameters, we varied the local density of the SiH4 gas by changing its stagnation pressure on the injection line over the range of 0.1–2.0 bar. We observed that nanocrystals were synthesized in a size range from ∼2 to ∼50 nm with monocrystalline morphology. Smaller nanocrystals (∼2–6 nm) with narrower size distributions and with higher number densities were synthesized with an increase of the SiH4 gas-phase density. We related this observation to the rapid depletion of the number density of the molecules, ions, and radicals in the plasma during nanocrystal growth, which can primarily occur via nucleation with no significant subsequent coagulation. In addition, in our remote plasma environment, rapid cooling of the gas in the particle growth zone from ∼1500 to ∼400 K significantly reduces the coalescence...

Analysis of temporal evolution of quantum dot surface chemistry by surface-enhanced Raman scattering

Surface enhanced Raman spectroscopy (SERS) was used to probe the surface chemistry of chlorineterminated silicon nanocrystal (Si-NC) surfaces in an air-free environment. SERS effect was observed from the thin films of AgxO using 514 nm laser wavelength. When a monolayer of Si-NCs were spincoated on AgxO SERS substrates, a very clear signal of surface states, including Si−Clx, and Si−Hx were observed. Upon air-exposure, we observed the temporal reduction of Si−Clx peak intensity, and a development of oxidation-related peak intensities, like Si−Ox and Si−O−Hx. In addition, first, second and third order transverse optical (TO) modes of Si-NCs were also observed at 519, 1000 and 1600 cm−1, respectively. As a comparison, Raman analysis of a thick film (> 200 nm) of Si-NCs deposited on ordinary glass substrates were performed. This analysis only demonstrated the first TO mode of Si-NCs, and the all the other features originated from SERS enhancement did not appear in the spectrum. These results conclude that, SERS is not only capable of single-molecule detection, but also a powerful technique for monitoring the surface chemistry of nanoparticles.Temporal evolution of surface chemistry during oxidation of silicon quantum dot (Si-QD) surfaces were probed using surface-enhanced Raman scattering (SERS). A monolayer of hydrogen and chlorine terminated plasma-synthesized Si-QDs were spin-coated on silver oxide thin films. A clearly enhanced signal of surface modes, including Si-Clx and Si-Hx modes were observed from as-synthesized Si-QDs as a result of the plasmonic enhancement of the Raman signal at Si-QD/silver oxide interface. Upon oxidation, a gradual decrease of Si-Clx and Si-Hx modes, and an emergence of Si-Ox and Si-O-Hx modes have been observed. In addition, first, second and third transverse optical modes of Si-QDs were also observed in the SERS spectra, revealing information on the crystalline morphology of Si-QDs. An absence of any of the abovementioned spectral features, but only the first transverse optical mode of Si-QDs from thick Si-QD films validated that the spectral features observed from Si-QDs on silver oxide thin films are originated from the SERS effect. These results indicate that real-time SERS is a powerful diagnostic tool and a novel approach to probe the dynamic surface/interface chemistry of quantum dots, especially when they involve in oxidative, catalytic, and electrochemical surface/interface reactions.

Improved size distribution control of silicon nanocrystals in a spatially confined remote plasma

This work demonstrates how to improve the size distribution of silicon nanocrystals (Si-NCs) synthesized in a remote plasma, in which the flow dynamics and the particular chemistry initially resulted in the formation of small (2–10 nm) and large (50–120 nm) Si-NCs. Plasma consists of two regions: an axially expanding central plasma beam and a background region around the expansion. Continuum fluid dynamics simulations demonstrate that a significant mass flow occurs from the central beam to the background region. This mass flow can be gradually reduced upon confinement of the central beam, preventing the mass transport to the background region. Transmission electron microscopy and Raman spectroscopy analyses demonstrate that the volume fraction of large Si-NCs decreases from ~77% to below 45% in parallel with the decrease of mass flow to the background region upon confinement, which indicates that large Si-NCs are synthesized in the background and small Si-NCs are synthesized in the central beam. Spatially resolved ion flux analyses demonstrate that the ions are localized in the central beam despite the mass flow to the background, indicating that the formation of small Si-NCs is governed by ion-assisted growth while the formation of large Si-NCs is governed by radical-neutral-assisted growth in the absence of ions. According to these observations, a better uniformity in the size distribution of Si-NCs can be obtained by creating a more uniform plasma flow and controlling the density of plasma species in the plasma.

An analytical force balance model for dust particles with size up to several Debye lengths

In this study, we developed a revised stationary force balance model for particles in the regime a/λD<10. In contrast to other analytical models, the pressure and dipole force were included too, and for anisotropic plasmas, a novel contribution to the dipole moment was derived. Moreover, the Coulomb logarithm and collection cross-section were modified. The model was applied on a case study where carbon dust is formed near the plasma sheath in the linear plasma device Pilot-PSI. The pressure force and dipole force were found to be significant. By tracing the equilibrium position, the particle radius was determined at which the particle deposits. The obtained particle radius agrees well with the experimentally obtained size and suggests better agreement as compared to the unrevised model.