Optimal surface functionalization of silicon quantum dots

Controlling Microarray Feature Spreading and Response Stability on Porous Silicon Platforms by Using Alkene-Terminal Ionic Liquids and UV Hydrosilylation

In an attempt to develop reversible sensors based on ionic liquid/porous silicon (IL/pSi) platforms, we introduce an approach using task-specific, alkene-terminal ILs (AT-ILs) for direct grafting to the hydrogen-passivated as prepared-pSi (ap-pSi) surface via UV-hydrosilylation to address previous shortcomings associated with IL pattern impermanence (i.e., spread). By employing photoluminescence emission (PLE) and Fourier-transform infrared (FT-IR) imaging measurements, we demonstrate that the covalent grafting of AT-ILs onto the ap-pSi surface via photochemical hydrosilylation not only mitigates such feature spreading but also greatly improves PLE pattern stability. Significantly, we have discovered that, upon hydrosilylation, the resulting contact pin printed IL features remain stable to repeated challenges by toluene vapors, demonstrating the utility of AT-IL hydrosilylation for producing high-fidelity microarray features on pSi toward robust optical sensory microarrays.

A dipole-dipole interaction tuning the photoluminescence of silicon quantum dots in a water vapor environment

The optical properties of silicon quantum dots (Si QDs) depend on the working conditions, which are critical for their application in optoelectronic devices and fluorescent tags. However, how a humid environment, most common in daily life, influences the photoluminescence (PL) of Si QDs has not been fully understood yet. Herein, we applied time-dependent density functional calculations to show that the adsorption of water molecules would exhibit distinct effects on the PL spectra of Si QDs as a function of size. In particular, the PL of Si QDs presents dual band emission with the adsorption of the cyclic water trimer (H2O)3 under common humid conditions, completely different from the PL of Si QDs under other conditions. The transition dipole moment decomposition analysis shows that the additional emission peak originates from the single Si-Si stretched bond of Si QDs induced by the dipole-dipole interaction between the cyclic water trimer and Si QDs. Moreover, the PL characteristics are size dependent. As the size increases from Si17H24 (the diameter of 0.6 nm) to Si52H52 (1.4 nm), the dipole-dipole interaction energy between (H2O)3 and Si QDs rapidly decreases from 19.1 × 10-22 J to 6.0 × 10-26 J, resulting in a single peak of PL of (H2O)3 adsorption on Si52H52. This study not only gives a deep understanding of PL of Si QDs under humid conditions, but also provides a new perspective on the development of optical devices based on Si QDs.

Tuning Confinement in Colloidal Silicon Nanocrystals with Saturated Surface Ligands

The optical properties of silicon nanocrystals (Si NCs) are a subject of intense study and continued debate. In particular, Si NC photoluminescence (PL) properties are known to depend strongly on the surface chemistry, resulting in electron-hole recombination pathways derived from the Si NC band-edge, surface-state defects, or combined NC-conjugated ligand hybrid states. In this Letter, we perform a comparison of three different saturated surface functional groups-alkyls, amides, and alkoxides-on nonthermal plasma-synthesized Si NCs. We find a systematic and size-dependent high-energy (blue) shift in the PL spectrum of Si NCs with amide and alkoxy functionalization relative to alkyl. Time-resolved photoluminescence and transient absorption spectroscopies reveal no change in the excited-state dynamics between Si NCs functionalized with alkyl, amide, or alkoxide ligands, showing for the first time that saturated ligands-not only surface-derived charge-transfer states or hybridization between NC and low-lying ligand orbitals-are responsible for tuning the Si NC optical properties. To explain these PL shifts we propose that the atom bound to the Si NC surface strongly interacts with the Si NC electronic wave function and modulates the Si NC quantum confinement. These results reveal a potentially broadly applicable correlation between the optoelectronic properties of Si NCs and related quantum-confined structures based on the interaction between NC surfaces and the ligand binding group.

Communication: Photoinduced carbon dioxide binding with surface-functionalized silicon quantum dots

Nowadays, the search for efficient methods able to reduce the high atmospheric carbon dioxide concentration has turned into a very dynamic research area. Several environmental problems have been closely associated with the high atmospheric level of this greenhouse gas. Here, a novel system based on the use of surface-functionalized silicon quantum dots (sf-SiQDs) is theoretically proposed as a versatile device to bind carbon dioxide. Within this approach, carbon dioxide trapping is modulated by a photoinduced charge redistribution between the capping molecule and the silicon quantum dots (SiQDs). The chemical and electronic properties of the proposed SiQDs have been studied with a Density Functional Theory and Density Functional Tight-Binding (DFTB) approach along with a time-dependent model based on the DFTB framework. To the best of our knowledge, this is the first report that proposes and explores the potential application of a versatile and friendly device based on the use of sf-SiQDs for photochemically activated carbon dioxide fixation.

Excited state dynamics study of the self-trapped exciton formation in silicon nanosheets

After excitation to S₁ (1), the exciton takes ∼450–850 femtoseconds to relax into the self-trapped (ST) state (2) with the occurrence of strong localization and a large Stokes shift, due to the significant stretching of the Si–Si bonds.

Origin of Analyte-Induced Porous Silicon Photoluminescence Quenching

We report on gaseous analyte-induced photoluminescence (PL) quenching of porous silicon, as-prepared (ap-pSi) and oxidized (ox-pSi). By using steady-state and emission wavelength-dependent time-resolved intensity luminescence measurements in concert with a global analysis scheme, we find that the analyte-induced quenching is best described by a three-component static quenching model. In the model, there are blue, green, and red emitters (associated with the nanocrystallite core and surface trap states) that each exhibit unique analyte-emitter association constants and these association constants are a consequence of differences in the pSi surface chemistries.

Tight-binding approximations to time-dependent density functional theory - A fast approach for the calculation of electronically excited states

We propose a new method of calculating electronically excited states that combines a density functional theory based ground state calculation with a linear response treatment that employs approximations used in the time-dependent density functional based tight binding (TD-DFTB) approach. The new method termed time-dependent density functional theory TD-DFT+TB does not rely on the DFTB parametrization and is therefore applicable to systems involving all combinations of elements. We show that the new method yields UV/Vis absorption spectra that are in excellent agreement with computationally much more expensive TD-DFT calculations. Errors in vertical excitation energies are reduced by a factor of two compared to TD-DFTB.

Instrumentation for Reliably Determining Porous Silicon Photoluminescence Responses to Gaseous Analyte Vapors

We report new instrumentation for rapidly and reliably measuring the temperature-dependent photoluminescence response from porous silicon as a function of analyte vapor concentration. The new system maintains the porous silicon under inert conditions and it allows on-the-fly steady-state and time-resolved photoluminescence intensity and hyper-spectral measurements between 293 K and 450 K. The new system yields reliable data at least 100-fold faster in comparison to previous instrument platforms.

Contact Pin-Printing onto Porous Silicon for Creating Microarrays with High Chemical Diversity

We explore the size and spatial microheterogeneity of contact pin-printed spots formed on porous silicon (pSi). Glycerol was contact printed at room temperature onto as-prepared, hydrogen-passivated pSi (ap-pSi) using 50 or 200 µm diameter solid pins. The pSi was then subjected to a strong oxidizing environment (gaseous O₃) and washed to remove the glycerol masks. The glycerol-free regions were converted to oxidized pSi (ox-pSi); the glycerol-coated regions were protected from O₃, but not entirely. The final array is described as circularly shaped "ap-pSi" regions on a field of ox-pSi. When comparing the areas outside and inside the glycerol-masked pSi spots, one finds dramatic differences in the Si-O-Si, SiH_x (x = 1-3) and O_ySiH_x (y, x = 1-3) levels with a spatially dependent continuum of compositions across the spot diameter. Experimental conditions could be adjusted to tune the final ap-pSi spot diameter and edge widths from 90 µm to 520 µm and 20 µm to 130 µm, respectively. The resulting ap-pSi spot diameter is explained by using molecular kinetic theory and time-dependent glycerol imbibement into the pSi within a one-dimensional Darcy's law model.

Synthetic Developments of Nontoxic Quantum Dots

Semiconductor nanocrystals, or quantum dots (QDs), are candidates for biological sensing, photovoltaics, and catalysis due to their unique photophysical properties. The most studied QDs are composed of heavy metals like cadmium and lead. However, this engenders concerns over heavy metal toxicity. To address this issue, numerous studies have explored the development of nontoxic (or more accurately less toxic) quantum dots. In this Review, we select three major classes of nontoxic quantum dots composed of carbon, silicon and Group I-III-VI elements and discuss the myriad of synthetic strategies and surface modification methods to synthesize quantum dots composed of these material systems.

Band gap modulation of Si-C binary core/shell nanowires by composition and ratio

Core/shell nanowires (CSNWs) composed of Si, C, and SiC are promising systems for optoelectronic devices. Through computational investigations, we find that the band gaps (Eg) of these nanowires can be controlled not only by changing their composition, but also by adjusting the core/shell thickness ratio. For Si/SiC or SiC/C CSNWs with a fixed total number of layers, the dependence of Eg on the core/shell thickness ratio shows a bowing effect. Eg can be tuned from a few eV all the way to zero. These investigations provide direction for designing optoelectronic devices based on Earth-abundant elements.

Colloidal silicon quantum dots: from preparation to the modification of self-assembled monolayers (SAMs) for bio-applications

Summarizes recent advances in the preparation, surface modification and bio-applications of silicon quantum dots.

Theoretical investigation on structural and electronic properties of organic dye C258 on TiO2(101) surface in dye-sensitized solar cells

Electron gets directly transferred from the HOMO of C258 to the bottom conduction band of TiO₂ in bidentate bridging adsorption mode.

An in-depth study linking the infrared spectroscopy and photoluminescence of porous silicon during ambient hydrogen peroxide oxidation

We carefully tailored a porous silicon (pSi) surface by oxidation with hydrogen peroxide (H2O2) to determine the time-dependent changes in nanocrystallite surface chemistries (e.g., Si-O-Si, SiH(x) [x = 1, 2], OySiH [y = 2, 3], and SiOH/H2O) and their influence on the pSi photoluminescence (PL). The relationship between infrared band amplitudes and PL intensity were evaluated under H2O2 and O3 (previously studied) oxidation. The pSi surface composition under O3 and H2O2 oxidation conditions tended to, save the O(y)SiH (y = 2, 3) species, approach similar values at the longest oxidation times studied, but they took very different paths in reaching these end points. Furthermore, the pSi surface compositions that exhibit maximum/minimum PL under each oxidant are very different.

Photoluminescence of silicon quantum dots in nanospheres

Si quantum dots (SiQDs) based nanospheres (SiNSs) were prepared via a novel synthetic strategy. These SiNSs were demonstrated to possess unique dot spacing dependent photoluminescence (PL) up-conversion and surface dependent (N modified surface) down-converted PL. It was demonstrated that a small distance between SiQDs (<5 nm) is the necessary condition for the PL up-conversion of SiNSs, while the surface state of SiQDs will affect the maximum emission wavelength and the PL intensity. The as-prepared SiNSs feature excellent aqueous dispersibility, and their optical properties were found to be stable enough in a specified temperature and pH range.

Surface passivation dependent photoluminescence from silicon quantum dot phosphors

We demonstrate wavelength-tunable, air-stable and nontoxic phosphor materials based on silicon quantum dots (SiQDs). The phosphors, which are composed of micrometer-size silicon particles with attached SiQDs, are synthesized by an electrochemical etching method under ambient conditions. The photoluminescence (PL) peak wavelength can be controlled by the SiQD size due to quantum confinement effect, as well as the surface passivation chemistry of SiQDs. The red-emitting phosphors have PL quantum yield equal to 17%. The SiQD-phosphors can be embedded in polymers and efficiently excited by 405 nm light-emitting diodes for potential general lighting applications.

High-throughput screening system for creating and assessing surface-modified porous silicon

A high-throughput screening system has been developed to rapidly produce, screen, and assess the usefulness of organically modified silane (ORMOSIL)-based xerogel films formed on the surface of porous silicon (pSi) surfaces. The ORMOSILs tested include methyltriethoxysilane, n-octyltriethoxysilane, n-hexyltriethoxysilane, n-propyltriethoxysilane, 2-cyanoethyltriethoxysilane, phenyltriethoxysilane, benzyltriethoxysilane, vinyltriethoxysilane, tetraethoxysilane, and hexafluoroethyltriethoxysilane. Xerogel microarrays were pin-printed on the surface of O(3) oxidized pSi using a computer-controlled robotic pin-printer. The fragile pSi required careful pin-printing parameter optimization to simultaneously ensure sufficient sol application and limit pin-induced damage. These multi-functional xerogel-pSi microarrays were exposed to harsh conditions (0.1 mM NaOH, 15 min) to determine the extent to which the xerogel protected the pSi. Microarray assessment included multispectral photoluminescence and infrared imaging. Results demonstrate that the more hydrophobic/nonpolar xerogel films (n-octyltriethoxysilane, n-hexyltriethoxysilane) protect the pSi surface the most and maintained the pSi photoluminescence. Also, unlike xerogel material doped with a reporter molecule, the uniformity of the printed feature plays a role in the protection of the pSi material underneath. Areas with thinner xerogel distributions allowed the permeation of NaOH whereas the thicker areas prohibit pSi exposure to NaOH.

Link between O2SiH infrared band amplitude and porous silicon photoluminescence during ambient O3 oxidation

We carefully evaluate how porous silicon (pSi) surface oxidation by ozone (O(3)) and the resulting changes in nanocrystallite surface chemistries (e.g., SiOSi, SiH(x) (x = 1-3), O(y)SiH (y = 1-2), and SiOH) influence the pSi photoluminescence (PL). We discover a relationship between the pSi PL and the O(2)SiH band amplitude.

Silicon Nanocrystal Functionalization: Analytic Fitting of DFTB Parameters

A density functional tight binding (DFTB) scheme has been applied to functionalized silicon nanocrystals. Using an analytic functional representation of DFTB parameters, the scheme has been used to compute the adsorption energies in the organic functionalization of reconstructed Si(100) and H-terminated Si(111) surfaces of hundreds-of-atoms nanocrystals. We adopt an ONIOM(QM:QM') approach that corrects the overbinding of DFTB, obtaining nice agreement with high-level reaction energies and structural configurations.

Asymmetric doping in silicon nanostructures: the impact of surface dangling bonds

We investigate peculiar dopant deactivation behaviors of Si nanostrucures with first principle calculations and reveal that surface dangling bonds (SDBs) on Si nanostructures could be fundamental obstacles in nanoscale doping. In contrast to bulk Si, as the size of Si becomes smaller, SDBs on Si nanostructures prefer to be charged and asymmetrically deactivate n- and p-type doping. The asymmetric dopant deactivation in Si nanostructures is ascribed to the preference for negatively charged SDBs as a result of a larger quantum confinement effect on the conduction band. On the basis of our results, we show that the control of the growth direction of silicon nanowire as well as surface passivation is very important in preventing dopant deactivation.

The influence of a single thiol group on the electronic and optical properties of the smallest diamondoid adamantane

At the nanoscale, the surface becomes pivotal for the properties of semiconductors due to an increased surface-to-bulk ratio. Surface functionalization is a means to include semiconductor nanocrystals into devices. In this comprehensive experimental study we determine in detail the effect of a single thiol functional group on the electronic and optical properties of the hydrogen-passivated nanodiamond adamantane. We find that the optical properties of the diamondoid are strongly affected due to a drastic change in the occupied states. Compared to adamantane, the optical gap in adamantane-1-thiol is lowered by approximately 0.6 eV and UV luminescence is quenched. The lowest unoccupied states remain delocalized at the cluster surface leaving the diamondoid's negative electron affinity intact.

Nanomaterials in fluorescence-based biosensing

Fluorescence-based detection is the most common method utilized in biosensing because of its high sensitivity, simplicity, and diversity. In the era of nanotechnology, nanomaterials are starting to replace traditional organic dyes as detection labels because they offer superior optical properties, such as brighter fluorescence, wider selections of excitation and emission wavelengths, higher photostability, etc. Their size- or shape-controllable optical characteristics also facilitate the selection of diverse probes for higher assay throughput. Furthermore, the nanostructure can provide a solid support for sensing assays with multiple probe molecules attached to each nanostructure, simplifying assay design and increasing the labeling ratio for higher sensitivity. The current review summarizes the applications of nanomaterials--including quantum dots, metal nanoparticles, and silica nanoparticles--in biosensing using detection techniques such as fluorescence, fluorescence resonance energy transfer (FRET), fluorescence lifetime measurement, and multiphoton microscopy. The advantages nanomaterials bring to the field of biosensing are discussed. The review also points out the importance of analytical separations in the preparation of nanomaterials with fine optical and physical properties for biosensing. In conclusion, nanotechnology provides a great opportunity to analytical chemists to develop better sensing strategies, but also relies on modern analytical techniques to pave its way to practical applications.