Surface Of Silicon Nanocrystals Research Articles

Luminescent silicon nanocrystals appended with photoswitchable azobenzene units.

Confinement of multiple azobenzene chromophores covalently linked at the surface of luminescent silicon nanocrystals preserves the photoswitching behavior and modulates the nanocrystal polarity. Concomitantly, the thermal Z→E isomerization is strongly accelerated and the nanocrystal luminescence is reduced by an energy transfer process resulting in photosensitized E→Z isomerization.

Characterization of Silicon Nanocrystal Surfaces by Multidimensional Solid-State NMR Spectroscopy

The chemical and photophysical properties of silicon nanocrystals (Si NCs) are strongly dependent on the chemical composition and structure of their surfaces. Here we use fast magic angle spinning (MAS) and proton detection to enable the rapid acquisition of dipolar and scalar 2D 1H–29Si heteronuclear correlation (HETCOR) solid-state NMR spectra and reveal a molecular picture of hydride-terminated and alkyl-functionalized surfaces of Si NCs produced in a nonthermal plasma. 2D 1H–29Si HETCOR and dipolar 2D 1H–1H multiple-quantum correlation spectra illustrate that resonances from surface mono-, di-, and trihydride groups cannot be resolved, contrary to previous literature assignments. Instead the 2D NMR spectra illustrate that there is large distribution of 1H and 29Si chemical shifts for the surface hydride species in both the as-synthesized and functionalized Si NCs. However, proton-detected 1H–29Si refocused INEPT experiments can be used to unambiguously differentiate NMR signals from the different surf...

Evolution of Microstructure and Incorporation of Excess Hydrogen During the Growth of Hydrogenated Polymorphous Silicon at High Rate

Hydrogenated polymorphous silicon (pm-Si:H) is a material consisting of a small volume fraction of nanocrystals embedded in an amorphous matrix, and which can be grown at high deposition rates by increasing the radio-frequency power. When grown at high deposition rates, pm-Si:H films show a shift of their infrared (IR) absorption stretching band peak to higher wavenumbers and a sudden increase in their optical bandgap. The IR absorption spectrum was analyzed by deconvolution into three bands, including a medium stretching mode positioned at 2030 cm−1, which has been attributed to Si–H bonds at silicon nanocrystal surfaces. Secondary ion mass spectrometry measurements confirmed that an excess of hydrogen is incorporated in pm-Si:H grown at high deposition rate, leading to a sharp increase in the optical bandgap. We suggest that this sharp increase can be used as a simple tool to detect the deterioration of material quality when using high deposition rate processes.

In situ IR-spectroscopy as a tool for monitoring the radical hydrosilylation process on silicon nanocrystal surfaces.

Among a variety of SiNC functionalization methods, radical initiated grafting is very promising due to its straightforward nature and low propensity to form surface oligomers. In the present study, we employed in situ IR spectroscopy in combination with visible light transmittance measurements to investigate the radical induced grafting process on the well-defined SiNCs. Our findings support the proposed model: unfunctionalized hydride-terminated SiNCs form agglomerates in organic solvents, which break up during the grafting process. However, clearing of the dispersion is not a valid indicator for complete surface functionalization. Furthermore, radical-initiated grafting reactions in which azobisisobutyronitrile (AIBN) is the initiator are strongly influenced by external factors including initiator concentration, grafting temperature, as well as substrate steric demand. The monomer concentration was proven to have a low impact on the grafting process. Based on these new insights an underlying mechanism could be discussed, offering an unprecedented view on the functionalization of SiNC surfaces via radical initiated hydrosilylation.

Instantaneous Functionalization of Chemically Etched Silicon Nanocrystal Surfaces

AbstractRemarkable advances in surface hydrosilylation reactions of C=C and C=O bonds on hydride‐terminated silicon have revolutionized silicon surface functionalization. However, existing methods for functionalizing hydride‐terminated Si nanocrystals (H‐SiNCs) require long reaction times and elevated temperatures. Herein, we report a room‐temperature method for functionalizing H‐SiNC surfaces within seconds by stripping outermost atoms on H‐SiNC surfaces with xenon difluoride (XeF2). Detailed analysis of the reaction byproducts by in situ NMR spectroscopy and GC‐MS provided unprecedented insight into NC surface composition and reactivity as well as the complex reaction mechanism of XeF2 activated hydrosilylation.

Instantaneous Functionalization of Chemically Etched Silicon Nanocrystal Surfaces.

Remarkable advances in surface hydrosilylation reactions of C=C and C=O bonds on hydride-terminated silicon have revolutionized silicon surface functionalization. However, existing methods for functionalizing hydride-terminated Si nanocrystals (H-SiNCs) require long reaction times and elevated temperatures. Herein, we report a room-temperature method for functionalizing H-SiNC surfaces within seconds by stripping outermost atoms on H-SiNC surfaces with xenon difluoride (XeF2 ). Detailed analysis of the reaction byproducts by in situ NMR spectroscopy and GC-MS provided unprecedented insight into NC surface composition and reactivity as well as the complex reaction mechanism of XeF2 activated hydrosilylation.

Exciton self‐trapped on Si–Si dimers on the surface of silicon nanocrystal: Experimental evidence

AbstractThe intensive photoluminescence (PL) band corresponding to the self‐trapped excitons (STEs) at surface Si–Si dimers has been observed for the multilayer periodical nanostructures (SiO/nc‐Si/) with average nanocrystal size of 2.5 and 3.5 nm. The maximum of the band is located at 1.06 m (1.17 eV) and does not depend on the nanocrystal size. The decay times of the PL band are 5.9 and 5.3 ms for the nanocrystal size of 2.5 and 3.5 nm, correspondingly. The obtained experimental results give a direct evidence of the exciton self‐trapping on the silicon nanocrystals surface predicted by G. Allan, C. Delerue, and M. Lannoo [Phys. Rev. B 76, 2961 (1996)].

Light-induced changes in silicon nanocrystal based solar cells: Modification of silicon–hydrogen bonding on silicon nanocrystal surface under illumination

Hydrogenated polymorphous silicon (pm-Si:H) is a material consisting of a small volume fraction of nanocrystals embedded in an amorphous matrix. pm-Si:H solar cells demonstrate interesting initial degradation behaviors such as rapid initial change in photovoltaic parameters and self-healing after degradation during light-soaking. The precise dynamics of the light-induced degradation was studied in a series of light-soaking experiments under various illumination conditions such as AM1.5G and filtered 570 nm yellow light. Hydrogen effusion experiment before and after light-soaking further revealed that the initial degradation of pm-Si:H solar cells originate from the modification of silicon–hydrogen bonding on the surface of silicon nanocrystals in pm-Si:H.

Grafting Poly(3‐hexylthiophene) from Silicon Nanocrystal Surfaces: Synthesis and Properties of a Functional Hybrid Material with Direct Interfacial Contact

AbstractHybrid functional materials (HFMs) comprised of semiconductor nanoparticles and conjugated polymers offer the potential of synergetic photophysical properties. We have developed HFMs based upon silicon nanocrystals (SiNCs) and the conductive polymer poly(3‐hexylthiophene) (SiNC@P3HT) by applying surface‐initiated Kumada catalyst transfer polycondensation (SI‐KCTP). One unique characteristic of the developed SiNC@P3HT is the formation of a direct covalent bonding between SiNCs and P3HT. The presented method for obtaining direct interfacial attachment, which is not accessible using other methods, may allow for the development of materials with efficient electronic communication at the donor–acceptor interfaces. Systematic characterization provides evidence of a core–shell structure, enhanced interfacial electron and/or energy transfer between the P3HT and SiNC components, as well as formation of a type‐II heterostructure.

Grafting Poly(3-hexylthiophene) from Silicon Nanocrystal Surfaces: Synthesis and Properties of a Functional Hybrid Material with Direct Interfacial Contact.

Hybrid functional materials (HFMs) comprised of semiconductor nanoparticles and conjugated polymers offer the potential of synergetic photophysical properties. We have developed HFMs based upon silicon nanocrystals (SiNCs) and the conductive polymer poly(3-hexylthiophene) (SiNC@P3HT) by applying surface-initiated Kumada catalyst transfer polycondensation (SI-KCTP). One unique characteristic of the developed SiNC@P3HT is the formation of a direct covalent bonding between SiNCs and P3HT. The presented method for obtaining direct interfacial attachment, which is not accessible using other methods, may allow for the development of materials with efficient electronic communication at the donor-acceptor interfaces. Systematic characterization provides evidence of a core-shell structure, enhanced interfacial electron and/or energy transfer between the P3HT and SiNC components, as well as formation of a type-II heterostructure.

Double stabilization of nanocrystalline silicon: a bonus from solvent

Double stabilization of the silicon nanocrystals was observed for the first time by 29Si and 13C MAS NMR spectroscopy. The role of solvent, 1,2-dimethoxyethane (glyme), in formation and stabilization of silicon nanocrystals as well as mechanism of modification of the surface of silicon nanocrystals by nitrogen-heterocyclic carbene (NHC) was studied in this research. It was shown that silicon nanocrystals were stabilized by the products of cleavage of the C–O bonds in ethers and similar compounds. The fact of stabilization of silicon nanoparticles with NHC ligands in glyme was experimentally detected. It was demonstrated that MAS NMR spectroscopy is rather informative for study of the surface of silicon nanoparticles but it needs very pure samples.

Determining the impact of hydrofluoric acid on surface states of as-prepared and chemically modified Si nanocrystals

The paper demonstrates an easy and cheap approach to chemical functionalization of silicon nanocrystal surface leading to enhancement of photoluminescence and electrical transport properties.

Silyl Radical Abstraction in the Functionalization of Plasma-Synthesized Silicon Nanocrystals

Many silicon nanostructures have exhibited favorable optical properties following surface functionalization with molecular groups through a silicon–carbon bond. Here, we show the mechanism of functionalization of silicon nanocrystals synthesized in a nonthermal radiofrequency plasma is fundamentally different than in other silicon systems. In contrast to hydrosilylation, where homolytic cleavage of Si–H surface bonds is typically a prerequisite to functionalization, we demonstrate the dominant initiation step for plasma-synthesized silicon nanocrystals is abstraction of a silyl radical, ·SiH3, and generation of radical at the silicon nanocrystal surface. We experimentally trap the abstracted silyl radical and show this initiation mechanism occurs for both radical- and thermally-initiated reactions of alkenes using complementary FTIR and 1H NMR spectroscopies. These data additionally indicate that silylsilylation, addition of a Si–SiH3 group across an unsaturated hydrocarbon, competes with hydrosilylation. We also present a new empirical sizing curve as a convenient method to determine Si NC size from photoluminescence peak energy.

Radical Initiated Hydrosilylation on Silicon Nanocrystal Surfaces: An Evaluation of Functional Group Tolerance and Mechanistic Study.

Hydrosilylation is among the most common methods used for modifying silicon surface chemistry. It provides a wide range of surface functionalities and effective passivation of surface sites. Herein, we report a systematic study of radical initiated hydrosilylation of silicon nanocrystal (SiNC) surfaces using two common radical initiators (i.e., 2,2'-azobis(2-methylpropionitrile) and benzoyl peroxide). Compared to other widely applied hydrosilylation methods (e.g., thermal, photochemical, and catalytic), the radical initiator based approach is particle size independent, requires comparatively low reaction temperatures, and yields monolayer surface passivation after short reaction times. The effects of differing functional groups (i.e., alkene, alkyne, carboxylic acid, and ester) on the radical initiated hydrosilylation are also explored. The results indicate functionalization occurs and results in the formation of monolayer passivated surfaces.

Preparation and Properties of Nanocomposites of Silicon Oxide in Porous Silicon

The composition of the surface and electrical properties of silicon oxide nanocomposites of porous silicon were investigated by FTIR spectroscopy in the range of 400 – 4000 cm-1and by impedance spectroscopy in the frequency range of 25 Hz – 1 MHz. It was revealed the different dispersion of electrical capacitance in different frequency bands and the complex relaxation processes that affect the electrical transport properties of nanocomposites. The correlations between the IR absorption spectra of silicon oxide composites and processes of charge transfer was defined. On the basis of analysis of spectra of thermal stimulated depolarization it was shown that oxidation of the surface of porous silicon nanocrystals gives rise to changing the density of states in different energy ranges.

Nonradiative Recombination via Conical Intersections Arising at Defects on the Oxidized Silicon Surface

Nonradiative recombination of excitations in semiconductors limits the performance of photovoltaics, light-emitting diodes, photocatalysts, and other devices. Herein we investigate the role that two known defects on the oxidized surface of silicon play in nonradiative recombination in silicon nanocrystals. We apply ab initio multiple spawning and multireference electronic structure methods to model the nonradiative processes which follow excitation of two cluster models of silicon epoxide defects that differ in the oxidation state of their respective silicon atoms. We find conical intersections in both clusters, and these intersections are found to be accessible at energies corresponding to visible wavelengths. In both cases, photochemical opening of the epoxide ring precedes nonradiative decay. These results support the hypothesis that conical intersections associated with specific defect structures on the oxidized surface of silicon nanocrystals facilitate nonradiative recombination. Discussion regarding how this hypothesis can be tested experimentally is presented.

Phenomenological model of photoluminescence degradation and photoinduced defect formation in silicon nanocrystal ensembles under singlet oxygen generation

We propose a phenomenological model to explain photoluminescence degradation of silicon nanocrystals under singlet oxygen generation in gaseous and liquid systems. The model considers coupled rate equations, which take into account the exciton radiative recombination in silicon nanocrystals, photosensitization of singlet oxygen generation, defect formation on the surface of silicon nanocrystals as well as quenching processes for both excitons and singlet oxygen molecules. The model describes well the experimentally observed power law dependences of the photoluminescence intensity, singlet oxygen concentration, and lifetime versus photoexcitation time. The defect concentration in silicon nanocrystals increases by power law with a fractional exponent, which depends on the singlet oxygen concentration and ambient conditions. The obtained results are discussed in a view of optimization of the photosensitized singlet oxygen generation for biomedical applications.

Self-trapped excitons on the surface of Si nanocrystals in SiO2

The experimental data recently obtained by the femtosecond pump-probe spectroscopy technique have shown that the self-trapped exciton (STE) state plays a key role in the dynamics of hot excitons in photoexcited silicon nanocrystals embedded in a SiO matrix. We present a theoretical model of excitons self-trapped on Si–O bounds at the silicon nanocrystal surface. The thermally activated transition from the STE state to the nanocrystal has been studied. The nonradiative multiphonon recombination of the STE initiated by the interaction with vibrations of local dipoles in a amorphous SiO matrix has been considered, too. The relaxation process of “hot” carriers localized in Si nanocrystals has been modulated using Monte Carlo simulation. We have demonstrated that effective exchange between the STE states and “hot” free exciton states opens the new way of the energy relaxation and leads to the wide energy distribution of the hot carriers during 10–100 ps after excitation.

Oxygen passivation of silicon nanocrystals: Influences on trap states, electron mobility, and hybrid solar cell performance

Surface quality of nanostructures has a significant influence on related device performance. In this study, a large number of dangling bonds are detected on the silicon nanocrystal (Si NC) surface after surface modification. These dangling bonds work as carrier traps and degrade particle electrical properties. Therefore, controlled particle oxidization is proposed as a substitution method for surface passivation, which can reduce carrier traps easily and effectively. Electron mobility is improved dramatically after a 12-hour controlled oxidation, and as a result, Si NC/PTB7 hybrid solar cells (HSC) achieves an efficiency of 3.6%; this is more than twice the efficiency of devices fabricated with fresh Si NCs (without passivation), and the highest efficiency for Si NC-based HSCs reported to date. However, excessive oxidation should be avoided because it introduces an oxygen rich layer on the particle surface, which blocks carrier transport and deteriorates electrical properties inversely.

Size-independent organosilane functionalization of silicon nanocrystals using Wilkinson’s catalyst

A low-temperature size-independent method for modifying the surface chemistry of the silicon nanocrystal (SiNC) surface is reported. Wilkinson’s catalyst has been applied to accelerate the dehydrogenative coupling reaction between organosilane molecules and hydride-terminated SiNCs. Two strategies and multiple organosilanes were evaluated to study surface modification efficiency. During the investigations, it was determined that surface functionalization efficiency showed some dependence upon the organic modifier in question. The comparatively low reactivity of octadecyldimethyl silanes may result from the formation of “Si–Rh–Si” intermediates hindered by the steric bulk of the silanes when reacted with activated SiNCs. Quenching of the SiNC-based photoluminescence is observed and the origin of this phenomenon has been attributed to the present of trace rhodium on SiNCs detected using X-ray photoelectron spectroscopy.