Rebecca J. Anthony

High-efficiency silicon nanocrystal light-emitting devices

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.

Hybrid Silicon Nanocrystal−Organic Light-Emitting Devices for Infrared Electroluminescence

We demonstrate hybrid inorganic-organic light-emitting devices with peak electroluminescence (EL) at a wavelength of 868 nm using silicon nanocrystals (SiNCs). An external quantum efficiency of 0.6% is realized in the forward-emitted direction, with emission originating primarily from the SiNCs. Microscopic characterization indicates that complete coverage of the SiNCs on the conjugated polymer hole-transporting layer is required to observe efficient EL.

An All-Gas-Phase Approach for the Fabrication of Silicon Nanocrystal Light-Emitting Devices

We present an all-gas-phase approach for the fabrication of nanocrystal-based light-emitting devices. In a single reactor, silicon nanocrystals are synthesized, surface-functionalized, and deposited onto substrates precoated with a transparent electrode. Devices are completed by evaporation of a top metal electrode. The devices exhibit electroluminescence centered at a wavelength of λ = 836 nm with a peak external quantum efficiency exceeding 0.02%. This all-gas-phase approach permits controlled deposition of dense, functional nanocrystal films suitable for application in electronic devices.

Tunability Limit of Photoluminescence in Colloidal Silicon Nanocrystals

Luminescent silicon nanocrystals (Si NCs) have attracted tremendous research interest. Their size dependent photoluminescence (PL) shows great promise in various optoelectronic and biomedical applications and devices. However, it remains unclear why the exciton emission is limited to energy below 2.1 eV, no matter how small the nanocrystal is. Here we interpret a nanosecond transient yellow emission band at 590 nm (2.1 eV) as a critical limit of the wavelength tunability in colloidal silicon nanocrystals. In the “large size” regime (d > ~3 nm), quantum confinement dominantly determines the PL wavelength and thus the PL peak blue shifts upon decreasing the Si NC size. In the “small size” regime (d < ~2 nm) the effect of the yellow band overwhelms the effect of quantum confinement with distinctly increased nonradiative trapping. As a consequence, the photoluminescence peak does not exhibit any additional blue shift and the quantum yield drops abruptly with further decreasing the size of the Si NCs. This finding confirms that the PL originating from the quantum confined core states can only exist in the red/near infrared with energy below 2.1 eV; while the blue/green PL originates from surface related states and exhibits nanosecond transition.

Temperature Dependent Photoluminescence of Size-Purified Silicon Nanocrystals

The photoluminescence (PL) of size-purified silicon nanocrystals is measured as a function of temperature and nanoparticle size for pure nanocrystal films and polydimethylsiloxane (PDMS) nanocomposites. The temperature dependence of the bandgap is the same for both sample types, being measurably different from that of bulk silicon because of quantum confinement. Our results also suggest weaker interparticle and environmental coupling in the nanocomposites, with enhanced PL and an unexpected dependence of lifetime on size for the pure nanocrystal films at low temperatures. We interpret these results through differences in the low-temperature size dependence of the ensemble nonradiative equilibrium constants. The response of the PDMS nanocomposites provides a consistent measure of local temperature through intensity, lifetime, and wavelength in a polymer-dispersed morphology suitable for biomedical applications, and we exploit this to fabricate a small-footprint fiber-optic cryothermometer. A comparison of the two sample types offers fundamental insight into the photoluminescent behavior of silicon nanocrystal ensembles.

Langmuir probe measurements of electron energy probability functions in dusty plasmas

Langmuir probe measurements in dusty plasmas is a challenge because particle and film deposition on the probe leads to contamination and distortion of the current–voltage characteristics. This problem is particularly acute while determining the electron energy probability function (EEPF) from the second derivative of the Langmuir probe current–voltage characteristics. Here, we present reliable EEPF measurements in a capacitively coupled argon–silane dusty plasma using a fast-scanning and shielded Langmuir probe. A solenoid-actuated shield covered the probe and the probe was exposed to the plasma only for short periods of time (less than 6 s) when the current–voltage characteristics were recorded during rapid voltage scans. This approach minimized probe surface contamination. In presence of dust (silicon nanoparticles) the electron density decreased and the electron temperature increased in comparison to a pristine argon plasma. While the population of lower energy electrons decreased in presence of dust, the high energy tail region overlapped throughout the experiment. Langmuir probe measurements were complemented with ion density measurements using a capacitive probe and ex situ examination of particles using electron microscopy.

Accurate determination of the size distribution of Si nanocrystals from PL spectra

Narrow size distribution of quantum dots (QDs) is needed for their application in photovoltaics but collection of such information is difficult. Many experiments have shown the photoluminescence (PL) spectrum of Si QDs will broaden and peak position is size dependent. However, there is still lack of quantitative analysis of such phenomenon. In this paper, a model is developed to fit the PL spectrum based on spontaneous emission and the size distribution of the QDs. With this model, we can quantitatively analyse the QD size and its distribution using the PL spectra only, saving the need of time consuming and destructive characterization methods such as transmission electron microscopy (TEM). The optical bandgap can be extracted naturally from this PL model. The size and distribution of the QD which are obtained by fitting the PL spectra are then confirmed by measurements using TEM and XRD.

Influence of the surface termination on the light emission of crystalline silicon nanoparticles.

The light emission properties of silicon crystalline nanoparticles (SiNPs) have been investigated using steady-state and time-resolved photoluminescence measurements carried out at 12 K and at room temperature. To enable a comparative study of the role of surface terminal groups on the optical properties, we investigated SiNPs-H ensembles with the same mean NP diameter but differing on the surface termination, namely organic-functionalized with 1-dodecene (SiNPs-C12) and H-terminated (SiNPs-H). We show that although the spectral dependence of the light emission is rather unaffected by surface termination, characterized by a single broad band peaking at ∼1.64 eV, both the exciton recombination lifetimes and quantum yields display a pronounced dependence on the surface termination. Exciton lifetimes and quantum yields are found to be significantly lower in SiNPs-H compared SiNPs-C12. This difference is due to distinct non-radiative recombination probabilities resulting from inter-NP exciton migration, which in SiNPs-C12 is inhibited by the energy barriers imposed by the bulky surface groups. The surface groups of organic-terminated SiPs are responsible for the inhibition of inter-NP exciton transfer, yielding a higher quantum yield compared to SiNPs-H. The surface oxidation of SiNPs-C12 leads to the appearance of a phenomenon of an exciton transference from to the Si core to oxide-related states that contribute to light emission. These excitons recombine radiatively, explaining why the emission quantum of the organic-terminated SiNPs is the same after surface oxidation of SiNPs-C12.

Aging of Silicon Nanocrystals on Elastomer Substrates: Photoluminescence Effects

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.

Silicon nitride-capped silicon nanocrystals via a nonthermal dual-plasma synthesis approach

Improving the photoluminescence quantum yields and air-stability of silicon nanocrystals is crucial to expanding their influence in optoelectronic devices and other burgeoning application areas. Here, a dual-plasma approach for the synthesis of silicon nanocrystals capped with silicon nitride is reported. The reactor consists of two plasma stages in series: a primary radiofrequency (rf) plasma for silicon nanocrystal growth from silane and argon gas followed by a secondary rf plasma for silicon nitride growth using nitrogen gas as the reactant. The core-shell nanocrystals were characterized using optical and structural analyses, and the plasma was characterized using optical emission spectroscopy. The resulting core-shell nanocrystals show a reduced susceptibility to ambient air oxidation as compared to bare silicon nanocrystals alone. This result is a step toward achieving highly efficient and air-stable photoluminescence from silicon nanocrystals while avoiding organic functionalization.Improving the photoluminescence quantum yields and air-stability of silicon nanocrystals is crucial to expanding their influence in optoelectronic devices and other burgeoning application areas. Here, a dual-plasma approach for the synthesis of silicon nanocrystals capped with silicon nitride is reported. The reactor consists of two plasma stages in series: a primary radiofrequency (rf) plasma for silicon nanocrystal growth from silane and argon gas followed by a secondary rf plasma for silicon nitride growth using nitrogen gas as the reactant. The core-shell nanocrystals were characterized using optical and structural analyses, and the plasma was characterized using optical emission spectroscopy. The resulting core-shell nanocrystals show a reduced susceptibility to ambient air oxidation as compared to bare silicon nanocrystals alone. This result is a step toward achieving highly efficient and air-stable photoluminescence from silicon nanocrystals while avoiding organic functionalization.