First-Principles Study of Fluorescence in Silver Nanoclusters

Mechanisms of efficient fluorescence from biocompatible, ligand-protected silver nanoclusters (AgNC) are explored with an atomistic model of an icosahedral shaped AgNC passivated with 12 cytosine molecules representing single-stranded DNA. Spin-resolved density-functional theory with varying constraints to the total charge was used as a simulation probe to explore the electronic structure and photoluminescence of AgNCs. Visible photoemission in AgNCs is modeled through a synergy of radiative and nonradiative photoinduced dynamics computed by a combination of density matrix and density functional methods with explicit treatment of spin polarization. The ab initio computed charge-to-total energy correlation, <i>E</i>_tot(<i>ΔN</i>), of the modeled AgNC shows an approximate 2.2 eV discontinuity at a charge of <i>ΔN</i> = 5, which correlates with the DFT calculated band gap and with concept of superatom with closed shell valence electron count [<i>PNAS</i> <b>2008</b>, <i>105</i>, 9157]. UV photoexcitation of this cationic model followed by cascade thermalizations toward the band edges is modeled using Redfield theory, and the corresponding time-integrated emission is calculated. Peak emission near 610 nm is found, consistent with experimentally reported PL in AgNCs. This work gives further insight into the recombination kinetics of AgNC and can be used to aid in tailoring their optical properties to maximize fluorescence efficiency and tunability

First-Principles Study of Fluorescence in Silver Nanoclusters

Mechanisms of efficient fluorescence from biocompatible, ligand-protected silver nanoclusters (AgNC) are explored with an atomistic model of an icosahedral shaped AgNC passivated with 12 cytosine molecules representing single-stranded DNA. Spin-resolved density-functional theory with varying constraints to the total charge was used as a simulation probe to explore the electronic structure and photoluminescence of AgNCs. Visible photoemission in AgNCs is modeled through a synergy of radiative and nonradiative photoinduced dynamics computed by a combination of density matrix and density functional methods with explicit treatment of spin polarization. The ab initio computed charge-to-total energy correlation, <i>E</i>_tot(<i>ΔN</i>), of the modeled AgNC shows an approximate 2.2 eV discontinuity at a charge of <i>ΔN</i> = 5, which correlates with the DFT calculated band gap and with concept of superatom with closed shell valence electron count [<i>PNAS</i> <b>2008</b>, <i>105</i>, 9157]. UV photoexcitation of this cationic model followed by cascade thermalizations toward the band edges is modeled using Redfield theory, and the corresponding time-integrated emission is calculated. Peak emission near 610 nm is found, consistent with experimentally reported PL in AgNCs. This work gives further insight into the recombination kinetics of AgNC and can be used to aid in tailoring their optical properties to maximize fluorescence efficiency and tunability

Unraveling Photodimerization of Cyclohexasilane from Molecular Dynamics Studies

Photoinduced reactions of a pair of cyclohexasilane (CHS) monomers are explored by time-dependent excited-state molecular dynamics (TDESMD) calculations. In TDESMD trajectories, one observes vivid reaction events including dimerization and fragmentation. A general reaction pathway is identified as (i) ring-opening formation of a dimer, (ii) rearrangement induced by bond breaking, and (iii) decomposition through the elimination of small fragments. The identified pathway supports the chemistry proposed for the fabrication of silicon-based materials using CHS as a precursor. In addition, we find dimers have smaller HOMO–LUMO gaps and exhibit a red shift and line-width broadening in the computed photoluminescence spectra compared with a pair of CHS monomers

Correction to “Nature of Record Efficiency Fluid-Processed Nanotube–Silicon Heterojunctions”

Correction to “Nature of Record Efficiency Fluid-Processed Nanotube–Silicon Heterojunctions

Nature of Record Efficiency Fluid-Processed Nanotube–Silicon Heterojunctions

Although there has been significant recent progress in improving performance, the precise classification of nanotube–silicon heterojunctions remains ambiguous. Here, we use type, chirality, and length-purified single-walled carbon nanotubes to clarify the nature of these devices. Our junctions are assembled from freestanding nanotube sheets that show remarkable stability in response to repeated crumpling and folding during fluid processing, making the films well suited to flexible platforms. Despite modest ideality factors, the best diodes meet or exceed state-of-the-art characteristics, but with a surprising dependence on sample type. The data further suggest that these devices can be simultaneously categorized as either Schottky or p–n junctions, and we use scaling arguments to model the behavior over a broad range of sheet resistance and film thickness in a manner that highlights the critical role of nanotube midgap states. Our results demonstrate how band gap engineering can optimize these devices while emphasizing the important role of the junction morphology

Abrupt Size Partitioning of Multimodal Photoluminescence Relaxation in Monodisperse Silicon Nanocrystals

Intrinsic constraints on efficient photoluminescence (PL) from smaller alkene-capped silicon nanocrystals (SiNCs) put limits on potential applications, but the root cause of such effects remains elusive. Here, plasma-synthesized colloidal SiNCs separated into monodisperse fractions reveal an abrupt size-dependent partitioning of multilevel PL relaxation, which we study as a function of temperature. Guided by theory and simulation, we explore the potential role of resonant phonon interactions with “minigaps” that emerge in the electronic density of states (DOS) under strong quantum confinement. Such higher-order structures can be very sensitive to SiNC surface chemistry, which we suggest might explain the common implication of surface effects in both the emergence of multimodal PL relaxation and the loss of quantum yield with decreasing nanocrystal size. Our results have potentially profound implications for optimizing the radiative recombination kinetics and quantum yield of smaller ligand-passivated SiNCs

Enhancing the Elasticity of Ultrathin Single-Wall Carbon Nanotube Films with Colloidal Nanocrystals

Thin bilayers of contrasting nanomaterials are ubiquitous in solution-processed electronic devices and have potential relevance to a number of applications in flexible electronics. Motivated by recent mesoscopic simulations demonstrating synergistic mechanical interactions between thin films of single-wall carbon nanotubes (SWCNTs) and spherical nanocrystal (NC) inclusions, we use a thin-film wrinkling approach to query the compressive mechanics of hybrid nanotube/nanocrystal coatings adhered to soft polymer substrates. Our results show an almost 2-fold enhancement in the Young modulus of a sufficiently thin SWCNT film associated with the presence of a thin interpenetrating overlayer of semiconductor NCs. Mesoscopic distinct-element method simulations further support the experimental findings by showing that the additional noncovalent interfaces introduced by nanocrystals enhance the modulus of the SWCNT network and hinder network wrinkling

Ensemble Brightening and Enhanced Quantum Yield in Size-Purified Silicon Nanocrystals

We report on the quantum yield, photoluminescence (PL) lifetime, and ensemble photoluminescent stability of highly monodisperse plasma-synthesized silicon nanocrystals (SiNCs) prepared though density-gradient ultracentrifugation in mixed organic solvents. Improved size uniformity leads to a reduction in PL line width and the emergence of entropic order in dry nanocrystal films. We find excellent agreement with the anticipated trends of quantum confinement in nanocrystalline silicon, with a solution quantum yield that is independent of nanocrystal size for the larger fractions but decreases dramatically with size for the smaller fractions. We also find a significant PL enhancement in films assembled from the fractions, and we use a combination of measurement, simulation, and modeling to link this “brightening” to a temporally enhanced quantum yield arising from SiNC interactions in ordered ensembles of monodisperse nanocrystals. Using an appropriate excitation scheme, we exploit this enhancement to achieve photostable emission

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