Luminescent Mechanism and Anti-Counterfeiting Application of Hydrophilic, Undoped Room-Temperature Phosphorescent Silicon Nanocrystals

Silicon nanocrystals (SiNCs) have attracted extensive attention in many advanced applications due to silicon's high natural abundance, low toxicity, and impressive optical properties. However, these applications are mainly focused on fluorescent SiNCs, little attention is paid to SiNCs with room-temperature phosphorescence (RTP) and their relative applications, especially water-dispersed ones. Herein, this work presents water-dispersible RTP SiNCs (UA-SiNCs) and their optical applications. The UA-SiNCs with a uniform particle size of 2.8 nm are prepared by thermal hydrosilylation between hydrogen-terminated SiNCs (H-SiNCs) and 10-undecenoic acid (UA). Interestingly, the resultant UA-SiNCs can exhibit tunable long-lived RTP with an average lifetime of 0.85 s. The RTP feature of the UA-SiNCs is confirmed to the n-π* transitions of their surface C═O groups. Subsequently, new dual-modal emissive UA-SiNCs-based ink is fabricated by blending with sodium alginate (SA) as the binder. The customized anticounterfeiting labels are also prepared on cellulosic substrates by screen-printing technique. As expected, UA-SiNCs/SA ink exhibits excellent practicability in anticounterfeiting applications. These findings will trigger the rapid development of RTP SiNCs, envisioning enormous potential in future advanced applications such as high-level anti-counterfeiting, information encryption, and so forth.

Hydrogenation of silicon-nanocrystals-embedded silicon oxide passivating contacts

We investigate the effect of hydrogen passivation of dangling bonds in silicon oxide passivating contacts with embedded silicon nanocrystals (NAnocrystalline Transport path in Ultra-thin dielectrics for REinforced passivation contact, NATURE contact). We first investigated the differences in electrical properties of the samples after hydrogen gas annealing and hydrogen plasma treatment (HPT). The results show that the NATURE contact was efficiently passivated by hydrogen after HPT owing to the introduction of hydrogen radicals into the structure. Furthermore, we examined the dependence of process parameters such as HPT temperature, duration, and H2pressure, on the electrical properties and hydrogen depth profiles. As a result, HPT at 500 °C, 15 min, and 0.5 Torr resulted in a large amount of hydrogen inside the NATURE contact and the highest implied open-circuit voltage of 724 mV. Contact resistivity and surface roughness hardly increased when HPT was performed under the optimized condition, which only improved the passivation performance without deteriorating the electron transport properties of the NATURE contact.

Semiempirical Two-Dimensional Model of the Bipolar Resistive Switching Process in Si-NCs/SiO2 Multilayers

In this work, the SET and RESET processes of bipolar resistive switching memories with silicon nanocrystals (Si-NCs) embedded in an oxide matrix is simulated by a stochastic model. This model is based on the estimation of two-dimensional oxygen vacancy configurations and their relationship with the resistive state. The simulation data are compared with the experimental current-voltage data of Si-NCs/SiO2 multilayer-based memristor devices. Devices with 1 and 3 Si-NCs/SiO2 bilayers were analyzed. The Si-NCs are assumed as agglomerates of fixed oxygen vacancies, which promote the formation of conductive filaments (CFs) through the multilayer according to the simulations. In fact, an intermediate resistive state was observed in the forming process (experimental and simulated) of the 3-BL device, which is explained by the preferential generation of oxygen vacancies in the sites that form the complete CFs, through Si-NCs.

Digital and Analog Resistive Switching Behavior in Si-NCs Embedded in a Si/SiO2 Multilayer Structure for Neuromorphic Systems

In this work, we report the digital and analog resistive-switching (RS) characteristics in a memristor based on silicon nanocrystals (Si-NCs) integrated into a complementary metal-oxide-semiconductor (MOS) structure. Si-NCs with a diameter of 5.48 ± 1.24 nm embedded in a SiO₂/Si-NCs/SiO₂ multilayer structure acts as an RS layer. These devices exhibit bipolar RS with an intermediate resistance step during SET and RESET processes, which is believed to lie in the Si-NCs layer acting as charge-trapping nodes. The endurance studies of about 70 DC cycles indicate an ON/OFF ratio of ~10⁶ and a retention time larger than 10⁴ s. Long-term potentiation (LTP, -2 V) and long-term depression (LTD, +4 V) are obtained by applying consecutive identical pulse voltages of 150 ms duration. The current value gradually increases/decreases (LTP/LTD) as the pulse number increases. Three consecutive identical pulses of -2 V/150 ms (LTP) separated by 5 and 15 min show that the last current value obtained at the end of each pulse train is kept, confirming an analog RS behavior. These characteristics provide a possible way to mimic biological synapse functions for applications in neuromorphic computing in Si-NCs-based CMOS structures.

Postproduction Approach to Enhance the External Quantum Efficiency for Red Light-Emitting Diodes Based on Silicon Nanocrystals

Despite bulk crystals of silicon (Si) being indirect bandgap semiconductors, their quantum dots (QDs) exhibit the superior photoluminescence (PL) properties including high quantum yield (PLQY > 50%) and spectral tunability in a broad wavelength range. Nevertheless, their low optical absorbance character inhibits the bright emission from the SiQDs for phosphor-type light emitting diodes (LEDs). In contrast, a strong electroluminescence is potentially given by serving SiQDs as an emissive layer of current-driven LEDs with (Si-QLEDs) because the charged carriers are supplied from electrodes unlike absorption of light. Herein, we report that the external quantum efficiency (EQE) of Si-QLED was enhanced up to 12.2% by postproduction effect which induced by continuously applied voltage at 5 V for 9 h. The active layer consisted of SiQDs with a diameter of 2.0 nm. Observation of the cross-section of the multilayer QLEDs device revealed that the interparticle distance between adjacent SiQDs in the emissive layer is reduced to 0.95 nm from 1.54 nm by “post-electric-annealing”. The shortened distance was effective in promoting charge injection into the emission layer, leading improvement of the EQE.

Study of the Effect of Nitric Acid in Electrochemically Synthesized Silicon Nanocrystals: Tunability of Bright and Uniform Photoluminescence

In this work, we show a correlation between the composition and the microstructural and optical properties of bright and uniform luminescent porous silicon (PSi) films. PSi films were synthesized by electrochemical etching using nitric acid in an electrolyte solution. PSi samples synthesized with nitric acid emit stronger (up to six-fold greater) photoluminescence (PL) as compared to those obtained without it. The PL peak is shifted from 630 to 570 nm by changing the concentration ratio of the HF:HNO₃:(EtOH-H₂O) electrolyte solution, but also shifts with the excitation energy, indicating quantum confinement effects in the silicon nanocrystals (Si-NCs). X-ray photoelectron spectroscopy analysis shows a uniform silicon content in the PSi samples that emit the strongest PL. High-resolution transmission electron microscopy reveals that the Si-NCs in these PSi samples are about ~2.9 ± 0.76 nm in size and are embedded in a dense and stoichiometric SiO₂ matrix, as indicated by the Fourier transform infrared analysis. On the other hand, the PSi films that show PL of low intensity present an abrupt change in the silicon content depth and the formation of non-bridging oxygen hole center defects.

Two-Photon Excitation Spectroscopy of Silicon Quantum Dots and Ramifications for Bio-Imaging

Two-photon excitation in the near-infrared (NIR) of colloidal nanocrystalline silicon quantum dots (nc-SiQDs) with photoluminescence also in the NIR has potential opportunities in the field of deep biological imaging. Spectra of the degenerate two-photon absorption (2PA) cross section of colloidal nc-SiQDs are measured using two-photon excitation over a spectral range 1.46 < ℏω < 1.91 eV (wavelength 850 > λ > 650 nm) above the two-photon band gap E_g^(QD)/2, and at a representative photon energy ℏω = 0.99 eV (λ = 1250 nm) below this gap. Two-photon excited photoluminescence (2PE-PL) spectra of nc-SiQDs with diameters d = 1.8 ± 0.2 nm and d = 2.3 ± 0.3 nm, each passivated with 1-dodecene and dispersed in toluene, are calibrated in strength against 2PE-PL from a known concentration of Rhodamine B dye in methanol. The 2PA cross section is observed to be smaller for the smaller diameter nanocrystals, and the onset of 2PA is observed to be blue shifted from the two-photon indirect band gap of bulk Si, as expected for quantum confinement of excitons. The efficiencies of nc-SiQDs for bioimaging using 2PE-PL are simulated in various biological tissues and compared to efficiencies of other quantum dots and molecular fluorophores and found to be comparable or superior at greater depths.

Water-Soluble Luminescent Silicon Nanocrystals by Plasma-Induced Acrylic Acid Grafting and PEGylation

Luminescent silicon nanocrystals are promising nanomaterials for biomedical applications due to their unique optical properties and biocompatibility. Here, we demonstrate a two-step surface modification approach coupling gas-phase and liquid-phase methods to synthesize PEGylated acrylic acid grafted silicon nanocrystals with near-infrared emission in water and biological media. First, acrylic acid grafted silicon nanocrystals are synthesized by an all-gas-phase approach on a millisecond time scale, omitting high temperature and postpurification processes. Subsequently, room-temperature PEGylation is carried out with these acrylic acid grafted silicon nanocrystals, yielding stable colloidal dispersions in both water and high ionic strength Tyrode's buffer with 20-30 nm hydrodynamic diameters. The PEGylated silicon nanocrystals exhibit photoluminescence in the 650-900 nm near-IR window with quantum yields of ∼30% and ∼13% in deionized water and Tyrode's buffer, respectively, after a 7-day oxidation in water. The surface-functionalized Si NCs exhibit relatively small toxicity to MDA-MB-231 cells at concentrations relevant to bioimaging applications.

Effects of thermal annealing on the distribution of boron and phosphorus in p-i-n structured silicon nanocrystals embedded in silicon dioxide

Thermal annealing temperature and time dictate the microstructure of semiconductor materials such as silicon nanocrystals (Si NCs). Herein, atom probe tomography (APT) and density functional theory (DFT) calculations are used to understand the thermal annealing temperature effects on Si NCs grown in a SiO₂matrix and the distribution behaviour of boron (B) and phosphorus (P) dopant atoms. The APT results demonstrate that raising the annealing temperature promotes growth and increased P concentration of the Si NCs. The data also shows that the thermal annealing does not promote the incorporation of B atoms into Si NCs. Instead, B atoms tend to locate at the interface between the Si NCs and SiO₂matrix. The DFT calculations support the APT data and reveal that oxygen vacancies regulate Si NC growth and dopant distribution. This study provides the detailed microstructure of p-type, intrinsic, and n-type Si NCs with changing annealing temperature and highlights how B and P dopants preferentially locate with respect to the Si NCs embedded in the SiO₂matrix with the aid of oxygen vacancies. These findings will be useful towards future optoelectronic applications.

Surface-Anisotropic Janus Silicon Quantum Dots via Masking on 2D Silicon Nanosheets

Surface-anisotropic nanoparticles represent a new class of materials that shows potential in a variety of applications, including self-assembly, microelectronics, and biology. Here, the first synthesis of surface-anisotropic silicon quantum dots (SiQDs), obtained through masking on 2D silicon nanosheets, is presented. SiQDs are deposited on the 2D substrate, thereby exposing only one side of the QDs, which is functionalized through well-established hydrosilylation procedures. The UV-sensitive masking substrate is removed through UV-irradiation, which simultaneously initiates the hydrosilylation of a second substrate, thereby introducing a second functional group to the other side of the now free-standing SiQDs. This renders surface-anisotropic SiQDs that have two different functional groups on either side of the particle. This method can be used to introduce a variety of functional groups including hydrophilic and hydrophobic substrates, while the unique optoelectronic properties of the SiQDs remain unaffected. The anisotropic morphology of the QDs is confirmed through the aggregation behavior of amphiphilic Janus SiQDs at the interface of water and hexane. Additionally, anisotropic SiQDs are used to produce the first controlled (sub)monolayer of SiQDs on a gold wafer.

Silicon Quantum Dot-Polymer Fabry-Pérot Resonators with Narrowed and Tunable Emissions

Luminescent silicon nanoparticles have been widely recognized as an alternative for metal-based quantum dots (QDs) for optoelectronics partly because of the high abundance and biocompatibility of silicon. To date, the broad photoluminescence line width (often >100 nm) of silicon QDs has been a hurdle to achieving competitive spectral purity and incorporating them into light-emitting devices. Herein we report fabrication and testing of straightforward configuration of Fabry-Pérot resonators that incorporates a thin layer of SiQD-polymer hybrid/blend between two reflective silver mirrors; remarkably these devices exhibit up-to-14-fold narrowing of SiQD emission and achieve a spectral bandwidth as narrow as ca. 9 nm. Our polymer-based, SiQD-containing Fabry-Pérot resonators also provide convenient spectral tunability, can be prepared using a variety of polymer hosts and substrates, and enable rigid as well as flexible devices.

Fabricating Femtosecond Laser-Induced Periodic Surface Structures with Electrophysical Anisotropy on Amorphous Silicon

One-dimensional periodic surface structures were formed by femtosecond laser irradiation of amorphous hydrogenated silicon (a-Si:H) films. The a-Si:H laser processing conditions influence on the periodic relief formation as well as correlation of irradiated surfaces structural properties with their electrophysical properties were investigated. The surface structures with the period of 0.88 and 1.12 μm were fabricated at the laser wavelength of 1.25 μm and laser pulse number of 30 and 750, respectively. The orientation of the surface structure is defined by the laser polarization and depends on the concentration of nonequilibrium carriers excited by the femtosecond laser pulses in the near-surface region of the film, which affects a mode of the excited surface electromagnetic wave which is responsible for the periodic relief formation. Femtosecond laser irradiation increases the a-Si:H films conductivity by 3 to 4 orders of magnitude, up to 1.2 × 10⁻⁵ S∙cm, due to formation of Si nanocrystalline phase with the volume fraction from 17 to 28%. Dark conductivity and photoconductivity anisotropy, observed in the irradiated a-Si:H films is explained by a depolarizing effect inside periodic microscale relief, nonuniform crystalline Si phase distribution, as well as different carrier mobility and lifetime in plane of the studied samples along and perpendicular to the laser-induced periodic surface structures orientation, that was confirmed by the measured photoconductivity and absorption coefficient spectra.

Poly(N-Isopropylacrylamide)-Functional Silicon Nanocrystals for Thermosensitive Fluorescence Cellar Imaging

Silicon nanocrystals (Si NCs) have received surging interest as a type of quantum dot (QD) due to the availability of silicon in nature, tunable fluorescence emission properties and excellent biocompatibility. More importantly, compared with many group II-VI and III-V based QDs, they have low toxicity. Here, thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm)-functional Si NCs were firstly prepared for thermoresponsive detection of cancer cells. Si NCs were prepared under normal pressure with excellent water solubility. Then folic acid was bonded to the silicon nanocrystals through the reaction of amino and carboxyl groups for specific recognition of cancer cells. The folic-acid-modified silicon crystals (Si NCs-FA) could be modified by a one-pot copolymerization process into PNIPAAm nanospheres during the monomer polymerization process (i.e., Si NCs-FA-PNIPAAm) just by controlling the temperature below the lower critical solution temperature (LCST) and above the LCST. The results showed that the Si-FA-PNIAAm nanospheres exhibited not only reversible temperature-responsive on-off fluorescence properties, but also can be used as temperature indicators in cancer cells.

Enhancing Singlet Oxygen Generation in Conjugates of Silicon Nanocrystals and Organic Photosensitizers

Silicon nanocrystals (SiNCs) are regarded as a green and environmentally friendly material when compared with other semiconductor nanocrystals. Ultra-small SiNCs (with the size 4.6-5.2 nm) demonstrate strong UV absorption and photoluminescence in the near infrared (NIR) range with the high photoluminescence quantum yield (PLQY) up to 60%. In contrast to nanoporous silicon, ultra-small SiNCs do not possess an intrinsic ability to generate singlet oxygen (1O2). However, we demonstrate that SiNC-dye conjugates synthesized via microwave assistant hydrosilylation reaction produce 1O2 with moderate quantum yield (ΦΔ) up to 27% in cyclohexane. These interesting results were obtained via measurements of singlet oxygen phosphorescence at 1,270 nm. SiNCs play an important role in the production of singlet oxygen as SiNCs harvest UV and blue radiation and transfer absorbed energy to a triplet state of the attached dyes. It increases the population of the triplet states and leads to the enhancement of the singlet oxygen generation. Simultaneously, the SiNC-dye conjugates demonstrate NIR luminescence with the PLQY up to 22%. Thus, the luminescence behavior and photosensitizing properties of the SiNC-dye conjugates can attract interest as a new multifunctional platform in the field of bio-applications.

Bright Silicon Nanocrystals from a Liquid Precursor: Quasi-Direct Recombination with High Quantum Yield

Silicon nanocrystals (SiNCs) with bright bandgap photoluminescence (PL) are of current interest for a range of potential applications, from solar windows to biomedical contrast agents. Here, we use the liquid precursor cyclohexasilane (Si₆H₁₂) for the plasma synthesis of colloidal SiNCs with exemplary core emission. Through size separation executed in an oxygen-shielded environment, we achieve PL quantum yields (QYs) approaching 70% while exposing intrinsic constraints on efficient core emission from smaller SiNCs. Time-resolved PL spectra of these fractions in response to femtosecond pulsed excitation reveal a zero-phonon radiative channel that anticorrelates with QY, which we model using advanced computational methods applied to a 2 nm SiNC. Our results offer additional insight into the photophysical interplay of the nanocrystal surface, quasi-direct recombination, and efficient SiNC core PL.

Si Nanocrystals/ZnO Nanowires Hybrid Structures as Immobilized Photocatalysts for Photodegradation

Numerous semiconductor-based hybrid nanostructures have been studied for improved photodegradation performance resulting from their broadband optical response and enhanced charge separation/transport characteristics. However, these hybrid structures often involve elements that are rare or toxic. Here, we present the synthesis and material characterization of hybrid nanostructures consisting of zinc oxide (ZnO) nanowires (NWs) and silicon nanocrystals (Si-NCs), both abundant and environmentally benign, and evaluate them for photodegradation performance under various illumination conditions. When incorporating Si-NCs into the vertically-aligned ZnO NWs immobilized on substrates, the resulting photocatalysts exhibited a narrowed band gap, i.e., more responsive to visible light, and enhanced charge separation at the interface, i.e., more reactive species produced for degradation. Consequently, the hybrid Si-NCs/ZnO-NWs displayed a superior photodegradability for methylene blue under UV and white light in comparison to the pristine ZnO NWs. Based on the optical measurements, we hypothesize the band structures of Si-NCs/ZnO-NWs and the potential mechanism for the improved photodegradability.

Silicon Quantum Dot Light Emitting Diode at 620 nm

Here we report a quantum dot light emitting diode (QLED), in which a layer of colloidal silicon quantum dots (SiQDs) works as the optically active component, exhibiting a strong electroluminescence (EL) spectrum peaking at 620 nm. We could not see any fluctuation of the EL spectral peak, even in air, when the operation voltage varied in the range from 4 to 5 V because of the possible advantage of the inverted device structure. The pale-orange EL spectrum was as narrow as 95 nm. Interestingly, the EL spectrum was narrower than the corresponding photoluminescence (PL) spectrum. The EL emission was strong enough to be seen by the naked eye. The currently obtained brightness (∼4200 cd/m²), the 0.033% external quantum efficiency (EQE), and a turn-on voltage as low as 2.8 V show a sufficiently high performance when compared to other orange-light-emitting Si-QLEDs in the literature. We also observed a parasitic emission from the neighboring compositional layer (i.e., the zinc oxide layer), and its intensity increased with the driving voltage of the device.

Silicon Nanocanyon: One-Step Bottom-Up Fabrication of Black Silicon via in-Lasing Hydrophobic Self-Clustering of Silicon Nanocrystals for Sustainable Optoelectronics

We report a novel one-step bottom-up fabrication method for multiscale-structured black Si, which is characterized by randomly distributed microscale Si layers covered with sub-100 nm protrusions with submicron boundary grooves. The unique multiscale structure, suggested as a "nanocanyon," effectively minimizes light reflection over a broad spectrum by diversifying the scattering routes from the nanotextured surface to the wide distributed boundary micronanoscale grooves. This structure was achieved by hydrophobic clustering and local aggregation of instantaneously melted Si nanocrystals on a glass substrate under laser irradiation. This method can replace the complicated conventional silicon processes, such as patterning for selective Si formation, texturing for improved absorption, and doping for modifying the electrical properties, because the proposed method obviates the need for photolithography, chemical etching, vacuum processes, and expensive wafers. Finally, black Si photosensor arrays were successfully demonstrated by a low-cost solution process and a laser growth sintering technique for microchannel fabrication. The results show the great potential of the proposed fabrication method for low-cost and sustainable production of highly sensitive optoelectronics and as an alternative to conventional wafer-based photosensor manufacturing techniques.

Dissipative Self-Assembly of Photoluminescent Silicon Nanocrystals

Solutions of silicon nanocrystals (SiNCs) are used in a diverse range of applications because of their tunable photoluminescence, biocompatibility, and the abundance of Si. In dissipative supramolecular materials, self-assembly of molecules or nanoparticles is driven by a chemical reaction network that irreversible consumes fuel. The properties of the emerging structures are controlled by the kinetics of the underlying chemical reaction network. Herein, we demonstrate the dissipative self-assembly of photoluminescent SiNCs driven by a chemical fuel. A chemical reaction induces self-assembly of the water-soluble SiNCs. However, the assemblies are transient, and when the chemical reaction network runs out of fuel, the SiNCs disassemble. The lifetime of the assemblies is controlled by the amount of fuel added. As an application of the transient supramolecular material, we demonstrate that the platform can be used to control the delayed uptake of the nanocrystals by mammalian cells.

Two-dimensional photonic crystals increasing vertical light emission from Si nanocrystal-rich thin layers

We have fabricated two-dimensional photonic crystals (PhCs) on the surface of Si nanocrystal-rich SiO₂ layers with the goal to maximize the photoluminescence extraction efficiency in the normal direction. The fabricated periodic structures consist of columns ordered into square and hexagonal pattern with lattice constants computed such that the red photoluminescence of Si nanocrystals (SiNCs) could couple to leaky modes of the PhCs and could be efficiently extracted to surrounding air. Samples having different lattice constants and heights of columns were investigated in order to find the configuration with the best performance. Spectral overlap of the leaky modes with the luminescence spectrum of SiNCs was verified experimentally by measuring photonic band diagrams of the leaky modes employing angle-resolved spectroscopy and also theoretically by computing the reflectance spectra. The extraction enhancement within different spatial angles was evaluated by means of micro-photoluminescence spectroscopy. More than 18-fold extraction enhancement was achieved for light propagating in the normal direction and up to 22% increase in overall intensity was obtained at the spatial collection angle of 14°.

Absence of free carriers in silicon nanocrystals grown from phosphorus- and boron-doped silicon-rich oxide and oxynitride

Phosphorus- and boron-doped silicon nanocrystals (Si NCs) embedded in silicon oxide matrix can be fabricated by plasma-enhanced chemical vapour deposition (PECVD). Conventionally, SiH4 and N2O are used as precursor gasses, which inevitably leads to the incorporation of ≈10 atom % nitrogen, rendering the matrix a silicon oxynitride. Alternatively, SiH4 and O2 can be used, which allows for completely N-free silicon oxide. In this work, we investigate the properties of B- and P-incorporating Si NCs embedded in pure silicon oxide compared to silicon oxynitride by atom probe tomography (APT), low-temperature photoluminescence (PL), transient transmission (TT), and current-voltage (I-V) measurements. The results clearly show that no free carriers, neither from P- nor from B-doping, exist in the Si NCs, although in some configurations charge carriers can be generated by electric field ionization. The absence of free carriers in Si NCs ≤5 nm in diameter despite the presence of P- or B-atoms has severe implications for future applications of conventional impurity doping of Si in sub-10 nm technology nodes.

Changes of the absorption cross section of Si nanocrystals with temperature and distance

The absorption cross section (ACS) of silicon nanocrystals (Si NCs) in single-layer and multilayer structures with variable thickness of oxide barriers is determined via a photoluminescence (PL) modulation technique that is based on the analysis of excitation intensity-dependent PL kinetics under modulated pumping. We clearly demonstrate that roughly doubling the barrier thickness (from ca. 1 to 2.2 nm) induces a decrease of the ACS by a factor of 1.5. An optimum separation barrier thickness of ca. 1.6 nm is calculated to maximize the PL intensity yield. This large variation of ACS values with barrier thickness is attributed to a modulation of either defect population states or of the efficiency of energy transfer between confined NC layers. An exponential decrease of the ACS with decreasing temperature down to 120 K can be explained by smaller occupation number of phonons and expansion of the band gap of Si NCs at low temperatures. This study clearly shows that the ACS of Si NCs cannot be considered as independent on experimental conditions and sample parameters.

Efficient Carrier Multiplication in Colloidal Silicon Nanorods

Auger recombination lifetimes, absorption cross sections, and the quantum yields of carrier multiplication (CM), or multiexciton generation (MEG), were determined for solvent-dispersed silicon (Si) nanorods using transient absorption spectroscopy (TAS). Nanorods with an average diameter of 7.5 nm and aspect ratios of 6.1, 19.3, and 33.2 were examined. Colloidal Si nanocrystals of similar diameters were also studied for comparison. The nanocrystals and nanorods were passivated with organic ligands by hydrosilylation to prevent surface oxidation and limit the effects of surface trapping of photoexcited carriers. All samples used in the study exhibited relatively efficient photoluminescence. The Auger lifetimes increased with nanorod length, and the nanorods exhibited higher CM quantum yield and efficiency than the nanocrystals with a similar band gap energy E_g. Beyond a critical length, the CM quantum yield decreases. Nanorods with the aspect ratio of 19.3 had the highest CM quantum yield of 1.6 ± 0.2 at 2.9E_g, which corresponded to a multiexciton yield that was twice as high as observed for the spherical nanocrystals.

Multi-chromatic silicon nanocrystals

Silicon nanocrystals (SiNCs) have great potential to become environmental friendly alternatives to heavy-metal containing nanocrystals for applications including medical imaging, lighting and displays. SiNCs exhibit excellent photostability, non-toxicity and abundant resources, but their often reported inefficient and spectrally limited light emission seriously impair their applications. Here we demonstrate a new method that converts SiNCs into an efficient and robust multi-chromatic phosphor. Using ~15 keV electron-beam irradiation of oxide-capped SiNCs, we introduce several types of color centers into the nanocrystal's oxide shell with efficient blue, green and red emission bands, together yielding warm-white photoluminescence, even for a single SiNC. Introduced centers are not native to the original system and we relate them to known defects in silica. Unlike in the silica host, however, here the centers are efficiently optically excitable. Provided further optimization and up-scaling of this method, e-beam irradiated SiNCs can be of great interest as white phosphors for applications such as LEDs.

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.

Orientationally Ordered Silicon Nanocrystal Cuboctahedra in Superlattices

Uniform silicon nanocrystals were synthesized with cuboctahedral shape and passivated with 1-dodecene capping ligands. Transmission electron microscopy, electron diffraction, and grazing incidence wide-angle and small-angle X-ray scattering show that these soft cuboctahedra assemble into face-centered cubic superlattices with orientational order. The preferred nanocrystal orientation was found to depend on the orientation of the superlattices on the substrate, indicating that the interactions with the substrate and assembly kinetics can influence the orientation of faceted nanocrystals in superlattices.

Impact of Silicon Nanocrystal Oxidation on the Nonmetallic Growth of Carbon Nanotubes

Carbon nanotube (CNT) growth has been demonstrated recently using a number of nonmetallic semiconducting and metal oxide nanoparticles, opening up pathways for direct CNT synthesis from a number of more desirable templates without the need for metallic catalysts. However, CNT growth mechanisms using these nonconventional catalysts has been shown to largely differ and reamins a challenging synthesis route. In this contribution we show CNT growth from partially oxidized silicon nanocrystals (Si NCs) that exhibit quantum confinement effects using a microwave plasma enhanced chemical vapor deposition (PECVD) method. On the basis of solvent and a postsynthesis frgamentation process, we show that oxidation of our Si NCs can be easily controlled. We determine experimentally and explain with theoretical simulations that the Si NCs morphology together with a necessary shell oxide of ∼1 nm is vital to allow for the nonmetallic growth of CNTs. On the basis of chemical analysis post-CNT-growth, we give insight into possible mechanisms for CNT nucleation and growth from our partially oxidized Si NCs. This contribution is of significant importance to the improvement of nonmetallic catalysts for CNT growth and the development of Si NC/CNT interfaces.

Cationic Silicon Nanocrystals with Colloidal Stability, pH-Independent Positive Surface Charge and Size Tunable Photoluminescence in the Near-Infrared to Red Spectral Range

In this report, the synthesis of a novel class of cationic quaternary ammonium-surface-functionalized silicon nanocrystals (ncSi) using a novel and highly versatile terminal alkyl halide-surface-functionalized ncSi synthon is described. The distinctive features of these cationic ncSi include colloidal stability, pH-independent positive surface charge, and size-tunable photoluminescence (PL) in the biologically relevant near-infrared-to-red spectral region. These cationic ncSi are characterized via a combination of high-resolution scanning transmission electron microscopy with energy-dispersive X-ray analysis, Fourier transform infrared, X-ray photoelectron, and photoluminescence spectroscopies, and zeta potential measurements.

Enhanced Luminescent Stability through Particle Interactions in Silicon Nanocrystal Aggregates

Close-packed assemblies of ligand-passivated colloidal nanocrystals can exhibit enhanced photoluminescent stability, but the origin of this effect is unclear. Here, we use experiment, simulation, and ab initio computation to examine the influence of interparticle interactions on the photoluminescent stability of silicon nanocrystal aggregates. The time-dependent photoluminescence emitted by structures ranging in size from a single quantum dot to agglomerates of more than a thousand is compared with Monte Carlo simulations of noninteracting ensembles using measured single-particle blinking data as input. In contrast to the behavior typically exhibited by the metal chalcogenides, the measured photoluminescent stability shows an enhancement with respect to the noninteracting scenario with increasing aggregate size. We model this behavior using time-dependent density functional theory calculations of energy transfer between neighboring nanocrystals as a function of nanocrystal size, separation, and the presence of charge and/or surface-passivation defects. Our results suggest that rapid exciton transfer from "bright" nanocrystals to surface trap states in nearest-neighbors can efficiently fill such traps and enhance the stability of emission by promoting the radiative recombination of slowly diffusing excited electrons.

Room-Temperature Reactivity Of Silicon Nanocrystals With Solvents: The Case Of Ketone And Hydrogen Production From Secondary Alcohols: Catalysis?

Although silicon nanoparticles dispersed in liquids are used in various applications ranging from biolabeling to hydrogen production, their reactivities with their solvents and their catalytic properties remain still unexplored. Here, we discovered that, because of their surface structures and mechanical strain, silicon nanoparticles react strongly with their solvents and may act as catalysts for the dehydrogenation, at room temperature, of secondary alcohols (e.g., isopropanol) into ketones and hydrogen. This catalytic reaction was monitored by gas chromatography, pH measurements, mass spectroscopy, and solid-state NMR. This discovery provides new understanding of the role played by silicon nanoparticles, and nanosilicon in general, in their reactivity in solvents in general, as well as being candidates in catalysis.

Quantum confined electron-phonon interaction in silicon nanocrystals

We study the micro-Raman spectra of colloidal silicon nanocrystals as a function of size, excitation wavelength, and excitation intensity. We find that the longitudinal optical (LO) phonon spectrum is asymmetrically broadened toward the low energy side and exhibits a dip or antiresonance on the high-energy side, both characteristics of a Fano line shape. The broadening depends on both nanocrystal size and Raman excitation wavelength. We propose that the Fano line shape results from interference of the optical phonon response with a continuum of electronic states that become populated by intraband photoexcitation of carriers. The asymmetry exhibits progressive enhancement with decreasing particle size and with increasing excitation energy for a given particle size. We compare our observations with those reported for p- and n-doped bulk Si, where Fano interference has also been observed, but we find opposite wavelength dependence of the asymmetry for the bulk and nanocrystalline Si. Our results have important implications for potentially controlling carrier energy relaxation channels in strongly confined Si nanocrystals.

Switching-on quantum size effects in silicon nanocrystals

The size-dependence of the absolute luminescence quantum yield of size-separated silicon nanocrystals reveals a "volcano" behavior, which switches on around 5 nm, peaks at near 3.7-3.9 nm, and decreases thereafter. These three regions respectively define: i) the transition from bulk to strongly quantum confined emissive silicon, ii) increasing confinement enhancing radiative recombination, and iii) increasing contributions favoring non-radiative recombination.

Size-dependent oxidation of monodisperse silicon nanocrystals with allylphenylsulfide surfaces

The synthesis and characterization of size-separated silicon nanocrystals functionalized with a heteroatom-substituted organic capping group, allylphenylsulfide, via photochemical hydrosilylation are described for the first time. These silicon nanocrystals form colloidally stable and highly photoluminescent dispersions in non-polar organic solvents with an absolute quantum yield as high as 52% which is 20% above that of the allylbenzene analogue. Solutions of the size-separated fractions are characterized over time to monitor the effect of aging in air by following the change of their photoluminescence and absolute quantum yields, supplemented by transmission electron microscopy.

Observing the morphology of single-layered embedded silicon nanocrystals by using temperature-stable TEM membranes

We use high-temperature-stable silicon nitride membranes to investigate single layers of silicon nanocrystal ensembles by energy filtered transmission electron microscopy. The silicon nanocrystals are prepared from the precipitation of a silicon-rich oxynitride layer sandwiched between two SiO2 diffusion barriers and subjected to a high-temperature annealing. We find that such single layers are very sensitive to the annealing parameters and may lead to a significant loss of excess silicon. In addition, these ultrathin layers suffer from significant electron beam damage that needs to be minimized in order to image the pristine sample morphology. Finally we demonstrate how the silicon nanocrystal size distribution develops from a broad to a narrow log-normal distribution, when the initial precipitation layer thickness and stoichiometry are below a critical value.

Size vs surface: tuning the photoluminescence of freestanding silicon nanocrystals across the visible spectrum via surface groups

The syntheses of colloidal silicon nanocrystals (Si-NCs) with dimensions in the 3-4 nm size regime as well as effective methodologies for their functionalization with alkyl, amine, phosphine, and acetal functional groups are reported. Through rational variation in the surface moieties we demonstrate that the photoluminescence of Si-NCs can be effectively tuned across the entire visible spectral region without changing particle size. The surface-state dependent emission exhibited short-lived excited-states and higher relative photoluminescence quantum yields compared to Si-NCs of equivalent size exhibiting emission originating from the band gap transition. The Si-NCs were exhaustively characterized using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transformed infrared spectroscopy (FTIR), and their optical properties were thoroughly investigated using fluorescence spectroscopy, excited-state lifetime measurements, photobleaching experiments, and solvatochromism studies.

Ultrafast Electrical Measurements of Isolated Silicon Nanowires and Nanocrystals

We simultaneously determined the charge carrier mobility and picosecond to nanosecond carrier dynamics of isolated silicon nanowires (Si NWs) and nanocrystals (Si NCs) using time-resolved terahertz spectroscopy. We then compared these results to data measured on bulk c-Si as a function of excitation fluence. We find >1 ns carrier lifetimes in Si NWs that are dominated by surface recombination with surface recombination velocities (SRV) between ∼1100-1700 cm s(-1) depending on process conditions. The Si NCs have markedly different decay dynamics. Initially, free-carriers are produced, but relax within ∼1.5 ps to form bound excitons. Subsequently, the excitons decay with lifetimes >7 ns, similar to free carriers produced in bulk Si. The isolated Si NWs exhibit bulk-like mobilities that decrease with increasing excitation density, while the hot-carrier mobilities in the Si NCs are lower than bulk mobilities and could only be measured within the initial 1.5 ps decay. We discuss the implications of our measurements on the utilization of Si NWs and NCs in macroscopic optoelectronic applications.

Size controlled synthesis of silicon nanocrystals using cationic surfactant templates

Alkyl-terminated silicon nanocrystals (Si NCs) are synthesized at room temperature by hydride reduction of silicon tetrachloride (SiCl₄) within inverse micelles. Highly monodisperse Si nanocrystals with average diameters ranging from 2 to 6 nm are produced by variation of the cationic quaternary ammonium salts used to form the inverse micelles. Transmission electron microscopy imaging shows that the NCs are highly crystalline, while FTIR spectra confirm that the NCs are passivated by covalent attachment of alkanes, with minimal surface oxidation. UV-vis absorbance and photoluminescence spectroscopy show significant quantum confinement effects, with moderate absorption in the UV spectral range, and a strong blue emission with a marked dependency on excitation wavelength. The photoluminescence quantum yield (Φ) of the Si NCs exhibits an inverse relationship with the mean NC diameter, with a maximum of 12% recorded for 2 nm NCs.

X-ray absorption near-edge structure anomalous behaviour in structures with buried layers containing silicon nanocrystals

Substructure and phase composition of silicon suboxide films containing silicon nanocrystals and implanted with carbon have been investigated by means of the X-ray absorption near-edge structure technique with the use of synchrotron radiation. It is shown that formation of silicon nanocrystals in the films' depth (more than 60 nm) and their following transformation into silicon carbide nanocrystals leads to abnormal behaviour of the X-ray absorption spectra in the elementary silicon absorption-edge energy region (100-104 eV) or in the silicon oxide absorption-edge energy region (104-110 eV). This abnormal behaviour is connected to X-ray elastic backscattering on silicon or silicon carbide nanocrystals located in the silicon oxide films depth.

Real-time activity bioassay of single osteoclasts using a silicon nanocrystal-impregnated artificial matrix

The lack of an in vitro real-time osteoclast (OC) activity assay has hampered mechanistic studies of bone resorption. Such an assay is developed, employing a hydroxyapatite matrix impregnated with alkyl-capped silicon nanocrystals, which is capable of monitoring the time-course of resorption by single osteoclasts. Resorption of the matrix by OC releases the nanocrystals, which are internalized by the cell and detected as an increase in OC luminescence. This particular choice of nanocrystals is motivated by their bright pH-independent luminescence, proportional to concentration, and by their rapid uptake without cytotoxicity. In this in vitro assay, OCs are inhibited by calcitonin (CT) and methyl-β-cyclodextrin (MCD), and stimulated by receptor activator of nuclear factor kappa-B ligand (RANKL) in the expected manner. The kinetics of the assay exhibit a lag phase representing cell attachment and commencement of resorption processes, followed by a growth of cell luminescence intensity, and the whole time-course is satisfactorily described by the logistic equation.

Chemical insight into the origin of red and blue photoluminescence arising from freestanding silicon nanocrystals

Silicon nanocrystals (Si NCs) are attractive functional materials. They are compatible with standard electronics and communications platforms and are biocompatible. Numerous methods have been developed to realize size-controlled Si NC synthesis. While these procedures produce Si NCs that appear identical, their optical responses can differ dramatically. Si NCs prepared using high-temperature methods routinely exhibit photoluminescence agreeing with the effective mass approximation (EMA), while those prepared via solution methods exhibit blue emission that is somewhat independent of particle size. Despite many proposals, a definitive explanation for this difference has been elusive for no less than a decade. This apparent dichotomy brings into question our understanding of Si NC properties and potentially limits the scope of their application. The present contribution takes a substantial step forward toward identifying the origin of the blue emission that is not expected based upon EMA predictions. It describes a detailed comparison of Si NCs obtained from three of the most widely cited procedures as well as the conversion of red-emitting Si NCs to blue emitters upon exposure to nitrogen-containing reagents. Analysis of the evidence is consistent with the hypothesis that the presence of trace nitrogen and oxygen even at the parts per million level in Si NCs gives rise to the blue emission.

Structural and optical characterization of pure Si-rich nitride thin films

The specific dependence of the Si content on the structural and optical properties of O- and H-free Si-rich nitride (SiNx>1.33) thin films deposited by magnetron sputtering is investigated. A semiempirical relation between the composition and the refractive index was found. In the absence of Si-H, N-H, and Si-O vibration modes in the FTIR spectra, the transverse and longitudinal optical (TO-LO) Si-N stretching pair modes could be unambiguously identified using the Berreman effect. With increasing Si content, the LO and the TO bands shifted to lower wavenumbers, and the LO band intensity dropped suggesting that the films became more disordered. Besides, the LO and the TO bands shifted to higher wavenumbers with increasing annealing temperature which may result from the phase separation between Si nanoparticles (Si-np) and the host medium. Indeed, XRD and Raman measurements showed that crystalline Si-np formed upon 1100°C annealing but only for SiNx<0.8. Besides, quantum confinement effects on the Raman peaks of crystalline Si-np, which were observed by HRTEM, were evidenced for Si-np average sizes between 3 and 6 nm. A contrario, visible photoluminescence (PL) was only observed for SiNx>0.9, demonstrating that this PL is not originating from confined states in crystalline Si-np. As an additional proof, the PL was quenched while crystalline Si-np could be formed by laser annealing. Besides, the PL cannot be explained neither by defect states in the bandgap nor by tail to tail recombination. The PL properties of SiNx>0.9 could be then due to a size effect of Si-np but having an amorphous phase.

In situ-grown hexagonal silicon nanocrystals in silicon carbide-based films

Silicon nanocrystals (Si-NCs) were grown in situ in carbide-based film using a plasma-enhanced chemical vapor deposition method. High-resolution transmission electron microscopy indicates that these nanocrystallites were embedded in an amorphous silicon carbide-based matrix. Electron diffraction pattern analyses revealed that the crystallites have a hexagonal-wurtzite silicon phase structure. The peak position of the photoluminescence can be controlled within a wavelength of 500 to 650 nm by adjusting the flow rate of the silane gas. We suggest that this phenomenon is attributed to the quantum confinement effect of hexagonal Si-NCs in silicon carbide-based film with a change in the sizes and emission states of the NCs.

Morphological, compositional, structural, and optical properties of Si-nc embedded in SiOx films

Structural, compositional, morphological, and optical properties of silicon nanocrystal (Si-nc) embedded in a matrix of non-stoichiometric silicon oxide (SiOx) films were studied. SiOx films were prepared by hot filament chemical vapor deposition technique in the 900 to 1,400°C range. Different microscopic and spectroscopic characterization techniques were used. The film composition changes with the growth temperature as Fourier transform infrared spectroscopy, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy reveal. High-resolution transmission electron microscopy supports the existence of Si-ncs with a diameter from 1 to 6.5 nm in the matrix of SiOx films. The films emit in a wide photoluminescent spectrum, and the maximum peak emission shows a blueshift as the growth temperature decreases. On the other hand, transmittance spectra showed a wavelength shift of the absorption border, indicating an increase in the energy optical bandgap, when the growth temperature decreases. A relationship between composition, Si-nc size, energy bandgap, PL, and surface morphology was obtained. According to these results, we have analyzed the dependence of PL on the composition, structure, and morphology of the Si-ncs embedded in a matrix of non-stoichiometric SiOx films.

Photothermal Response of Photoluminescent Silicon Nanocrystals

We demonstrate that silicon nanocrystals (Si-NCs) exhibiting relatively high near-IR photoluminescent quantum yields also exhibit a notable photothermal (PT) response. The PT effect has been quantified as a function of NC size, defect concentration, and irradiating energy, suggesting that the origin of the PT response is a combination of carrier thermalization and defect-mediated heating. The PT effect observed under NIR irradiation suggests that Si-NCs could find use in combined in vivo PL imaging and PT therapy.

Raman Spectroscopy of Oxide-Embedded and Ligand-Stabilized Silicon Nanocrystals

Oxide-embedded and oxide-free alkyl-terminated silicon (Si) nanocrystals with diameters ranging from 3 nm to greater than 10 nm were studied by Raman spectroscopy. For ligand-passivated nanocrystals, the zone center Raman-active mode of diamond cubic Si shifted to lower frequency with decreasing size, accompanied by asymmetric peak broadening, as extensively reported in the literature. The size dependence of the Raman peak shifts, however, was significantly more pronounced than previously reported or predicted by the RWL (Richter, Wang, and Ley) and bond polarizability models. In contrast, Raman peak shifts for oxide-embedded nanocrystals were significantly less pronounced as a result of the stress induced by the matrix.

Optical characterisation of silicon nanocrystals embedded in SiO2/Si3N4 hybrid matrix for third generation photovoltaics

Silicon nanocrystals with an average size of approximately 4 nm dispersed in SiO2/Si3N4 hybrid matrix have been synthesised by magnetron sputtering followed by a high-temperature anneal. To gain understanding of the photon absorption and emission mechanisms of this material, several samples are characterised optically via spectroscopy and photoluminescence measurements. The values of optical band gap are extracted from interference-minimised absorption and luminescence spectra. Measurement results suggest that these nanocrystals exhibit transitions of both direct and indirect types. Possible mechanisms of absorption and emission as well as an estimation of exciton binding energy are also discussed.

Colloidally stable silicon nanocrystals with near-infrared photoluminescence for biological fluorescence imaging

Luminescent silicon nanocrystals (ncSi) are showing great promise as photoluminescent tags for biological fluorescence imaging, with size-dependent emission that can be tuned into the near-infrared biological window and reported lack of toxicity. Here, colloidally stable ncSi with NIR photoluminescence are synthesized from (HSiO1.5)n sol-gel glasses and are used in biological fluorescence imaging. Modifications to the thermal processing conditions of (HSiO1.5)n sol-gel glasses, the development of new ncSi oxide liberation chemistry, and an appropriate alkyl surface passivation scheme lead to the formation of colloidally stable ncSi with photoluminescence centered at 955 nm. Water solubility and biocompatibility are achieved through encapsulation of the hydrophobic alkyl-capped ncSi within PEG-terminated solid lipid nanoparticles. Their applicability to biological imaging is demonstrated with the in-vitro fluorescence labelling of human breast tumor cells.

Alkyl passivation and amphiphilic polymer coating of silicon nanocrystals for diagnostic imaging

A method to produce biocompatible polymer-coated silicon nanocrystals for medical imaging is shown. Silica-embedded Si nanocrystals are formed by HSQ thermolysis. The nanocrystals are then liberated from the oxide and terminated with Si-H bonds by HF etching, followed by alkyl monolayer passivation by thermal hydrosilylation. The Si nanocrystals have an average diameter of 2.1 nm ± 0.6 nm and photoluminesce with a peak emission wavelength of 650 nm, which lies within the transmission window of 650-900 nm that is useful for biological imaging. The hydrophobic Si nanocrystals are then coated with an amphiphilic polymer for dispersion in aqueous media with the pH ranging between 7 and 10 and an ionic strength between 30 mM and 2 M, while maintaining a bright and stable photoluminescence and a hydrodynamic radius of only 20 nm. Fluorescence imaging of polymer-coated Si nanocrystals in biological tissue is demonstrated, showing the potential for in vivo imaging.

Photoelectric Properties of Silicon Nanocrystals/P3HT Bulk-Heterojunction Ordered in Titanium Dioxide Nanotube Arrays

A silicon nanocrystals (Si-ncs) conjugated-polymer-based bulk-heterojunction represents a promising approach for low-cost hybrid solar cells. In this contribution, the bulk-heterojunction is based on Si-ncs prepared by electrochemical etching and poly(3-hexylthiophene) (P3HT) polymer. Photoelectric properties in parallel and vertical device-like configuration were investigated. Electronic interaction between the polymer and surfactant-free Si-ncs is achieved. Temperature-dependent photoluminescence and transport properties were studied and the ratio between the photo- and dark-conductivity of 1.7 was achieved at ambient conditions. Furthermore the porous titanium dioxide (TiO(2)) nanotubes' template was used for vertical order of photosensitive Si-ncs/P3HT-based blend. The anodization of titanium foil in ethylene glycol-based electrolyte containing fluoride ions and subsequent thermal annealing were used to prepare anatase TiO(2) nanotube arrays. The arrays with nanotube inner diameter of 90 and 50 nm were used for vertical ordering of the Si-ncs/P3HT bulk-heterojunction.

Surface-Related States in Oxidized Silicon Nanocrystals Enhance Carrier Relaxation and Inhibit Auger Recombination

We have studied ultrafast carrier dynamics in oxidized silicon nanocrystals (NCs) and the role that surface-related states play in the various relaxation mechanisms over a broad range of photon excitation energy corresponding to energy levels below and above the direct bandgap of the formed NCs. Transient photoinduced absorption techniques have been employed to investigate the effects of surface-related states on the relaxation dynamics of photogenerated carriers in 2.8 nm oxidized silicon NCs. Independent of the excitation photon energy, non-degenerate measurements reveal several distinct relaxation regions corresponding to relaxation of photoexcited carriers from the initial excited states, the lowest indirect states and the surface-related states. Furthermore, degenerate and non-degenerate measurements at difference excitation fluences reveal a linear dependence of the maximum of the photoinduced absorption (PA) signal and an identical decay, suggesting that Auger recombination does not play a significant role in these nanostructures even for fluence generating up to 20 carriers/NC.