Optical Properties Of Silicon Nanocrystals Research Articles

Silicon Quantum Dots: Promising Theranostic Probes for the Future.

Nanotechnology has emerged as one of the leading research areas involving nanoscale manipulation of atoms and molecules. During the past decade, the growth of nanotechnology has been one of the most important developments that have taken place in the biomedical field. The new generation nanomaterials like Quantum dots are gaining much importance. Also, there is a growing interest in the development of nano-theranostics platforms in medical diagnostics, biomedical imaging, drug delivery, etc. Quantum dots are also known as nanoscale semiconductor crystals, with unique electronic and optical properties. Recently, silicon quantum dots are being studied extensively due to their less-toxic, inert nature and ease of surface modification. The silicon quantum dots (2-10nm) are comparatively stable, having optical properties of silicon nanocrystals. This review focuses on silicon quantum dots and their various biomedical applications like drug delivery regenerative medicine and tissue engineering. Also, the processes involved in their modification for various biomedical applications along with future aspects are discussed.

The influence of conjugated alkynyl(aryl) surface groups on the optical properties of silicon nanocrystals: photoluminescence through in-gap states

Developing new methods, other than size and shape, for controlling the optoelectronic properties of semiconductor nanocrystals is a highly desired target. Here we demonstrate that the photoluminescence (PL) of silicon nanocrystals (SiNCs) can be tuned in the range 685–800 nm solely via surface functionalization with alkynyl(aryl) (phenylacetylene, 2-ethynylnaphthalene, 2-ethynyl-5-hexylthiophene) surface groups. Scanning tunneling microscopy/spectroscopy on single nanocrystals revealed the formation of new in-gap states adjacent to the conduction band edge of the functionalized SiNCs. PL red-shifts were attributed to emission through these in-gap states, which reduce the effective band gap for the electron–hole recombination process. The observed in-gap states can be associated with new interface states formed via (–Si–C≡C–) bonds in combination with conjugated molecules as indicated by ab initio calculations. In contrast to alkynyl(aryl)s, the formation of in-gap states and shifts in PL maximum of the SiNCs were not observed with aryl (phenyl, naphthalene, 2-hexylthiophene) and alkynyl (1-dodecyne) surface groups. These outcomes show that surface functionalization with alkynyl(aryl) molecules is a valuable tool to control the electronic structure and optical properties of SiNCs via tuneable interface states, which may enhance the performance of SiNCs in semiconductor devices.

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.

Properties of silicon nanocrystals with boron and phosphorus doping fabricated via silicon rich oxide and silicon dioxide bilayers

Effects of boron and phosphorus doping on the structural, electrical, and optical properties of silicon nanocrystals in superlattice thin films were investigated. Silicon nanocrystals were fabricated via magnetron sputtering of stoichiometric silicon rich oxide and silicon dioxide bilayers followed by high temperature annealing at 1100 degrees Celsius. The characterization techniques used include: high-resolution transmission electron microscopy with energy filtering, grazing incidence x-ray diffraction, Raman, photoluminescence, and photothermal deflection spectroscopy, as well as electrical measurements. Results showed that phosphorus doping causes the loss of the bilayer structure and an increase in the average size of the silicon nanocrystals due to softening of the silicon dioxide matrix during post-sputter annealing. The result was a decrease in quantum confinement and a redshift in photoluminescence spectrum with an absorption profile similar to crystalline silicon. The undoped (intrinsic) sample maintained its bilayer structure and displayed stronger quantum confinement with higher photoluminescence peak energy and higher absorption coefficient. In-between, the boron doped sample was more similar structurally to the intrinsic sample, although merging between bilayers resulted in an extensive silicon nanocrystalline network. Optically, it displayed different effects due to photoluminescence quenching and free carrier absorption. Finally, both doped samples exhibited a decrease in electrical resistivity.

Structural and luminescence properties of silicon nanocrystals in colloidal solutions for bio applications

Luminescent silicon nanoparticles are promising not only for optoelectronics or photovoltaics, but also for biological and medical research, e.g., as luminescent markers, for drug delivery, or toxicity studies. For intracellular biological research, it is necessary to prepare water-based or isotonic colloidal solutions of nanoparticles which are stable and non-toxic. In this work, we compare structural and optical properties of silicon nanocrystals in colloidal solutions, obtained by two fundamentally different methods: A microplasma-based synthesis (a “bottom-up” technique) and porous silicon prepared by electrochemical etching of a silicon wafer (a “top–down” method). Low-pressure microwave plasma synthesis produces ∼5–8 nm large silicon nanocrystals while the porous siliconcontains clusters ∼60–70 nm in diameter, composed of nanometer-sized luminescent nanocrystals. However, both types of nanoparticles are prone to agglomeration, as was confirmed by dynamic light scattering and zeta potential measurements. We have attempted to stabilize the nanoparticles via modification of the their surface by methyl groups; however, the mechano-photo-chemical treatment procedure leading to coverage with methyl groups is less efficient in the case of plasma-synthesized nanoparticles than in porous silicon. The first attempt of steric stabilization of the colloidal solutions of the silicon nanoparticles was succesfully carried out too; good candidates for stabilization are bovine serum albumin and glycine.

Comparison of Silicon Nanocrystals Prepared by Two Fundamentally Different Methods.

This work compares structural and optical properties of silicon nanocrystals prepared by two fundamentally different methods, namely, electrochemical etching of Si wafers and low-pressure plasma synthesis, completed with a mechano-photo-chemical treatment. This treatment leads to surface passivation of the nanoparticles by methyl groups. Plasma synthesis unlike electrochemical etching allows selecting of the particle sizes. Measured sizes of the nanoparticles by dynamic light scattering show 3 and 20 nm for electrochemically etched and plasma-synthetized samples, respectively. Plasma-synthetized 20-nm particles do not exhibit photoluminescence due to absence of quantum confinement effect, and freshly appeared photoluminescence after surface passivation could indicate presence of organic molecules on the nanoparticle surface, luminescing instead of nanocrystal core. Electrochemically etched sample exhibits dramatic changes in photoluminescence during the mechano-photo-chemical treatment while no photoluminescence is observed for the plasma-synthetized one. We also used the Fourier transform infrared spectroscopy for comparison of the chemical changes happened during the treatment.

Synthesis and Properties of Luminescent Silicon Nanocrystal/Silica Aerogel Hybrid Materials

Exploiting the chemical compatibility of transparent high surface area silica aerogels and environmentally benign luminescent silicon nanocrystals (SiNCs) opens the door to a new class of silicon-based hybrid materials. Hydrophilic SiNCs of various sizes and surface groups that photoluminesce throughout the visible and NIR spectral regions were synthesized and incorporated into a tetramethyl orthosilicate containing solution to produce luminescent aerogels of varied transparency. Photoluminescence (PL) spectroscopy performed on solutions and aerogel monoliths containing SiNCs reveal that the optical properties of SiNCs are dependent on their size and surface chemistry. Furthermore, the incorporation of SiNCs into aerogels does not influence their PL response. The nitrogen adsorption–desorption measurements indicate that the physical properties of aerogels may be tailored by changing the SiNC size and surface chemistry. Adding to the appeal of the present work, PL quenching of SiNC-loaded aerogel upon expo...

Optical properties of silicon nanocrystals covered by periodic array of gold nanowires

Extinction and photoluminescence spectra are experimentally and theoretically studied for a periodic array of gold nanowires deposited on top of a dielectric substrate containing silicon nanocrysta ...

Structural and optical properties of silicon nanocrystals embedded in silicon carbide: Comparison of single layers and multilayer structures

The outstanding demonstration of quantum confinement in Si nanocrystals (Si NC) in a SiC matrix requires the fabrication of Si NC with a narrow size distribution. It is understood without controversy that this fabrication is a difficult exercise and that a multilayer (ML) structure is suitable for such fabrication only in a narrow parameter range. This parameter range is sought by varying both the stoichiometric SiC barrier thickness and the Si-rich SiC well thickness between 3 and 9nm and comparing them to single layers (SL). The samples processed for this investigation were deposited by plasma-enhanced chemical vapor deposition (PECVD) and subsequently subjected to thermal annealing at 1000–1100°C for crystal formation. Bulk information about the entire sample area and depth were obtained by structural and optical characterization methods: information about the mean Si NC size was determined from grazing incidence X-ray diffraction (GIXRD) measurements. Fourier-transform infrared spectroscopy (FTIR) was applied to gain insight into the structure of the Si–C network, and spectrophotometry measurements were performed to investigate the absorption coefficient and to estimate the bandgap E04. All measurements showed that the influence of the ML structure on the Si NC size, on the Si–C network and on the absorption properties is subordinate to the influence of the overall Si content in the samples, which we identified as the key parameter for the structural and optical properties. We attribute this behavior to interdiffusion of the barrier and well layers. Because the produced Si NC are within the target size range of 2–4nm for all layer thickness variations, we propose to use the Si content to adjust the Si NC size in future experiments.

Evolution of the Ultrafast Photoluminescence of Colloidal Silicon Nanocrystals with Changing Surface Chemistry

The role of surface species in the optical properties of silicon nanocrystals (SiNCs) is the subject of intense debate. Changes in photoluminescence (PL) energy following hydrosilylation of SiNCs with alkyl-terminated surfaces are most often ascribed to enhanced quantum confinement in the smaller cores of oxidized NCs or to oxygen-induced defect emission. We have investigated the PL properties of alkyl-functionalized SiNCs prepared using two related methods: thermal and photochemical hydrosilylation. Photochemically functionalized SiNCs exhibit higher emission energies than the thermally functionalized equivalent. While microsecond lifetime emission attributed to carrier recombination within the NC core was observed from all samples, much faster, size-independent nanosecond lifetime components were only observed in samples prepared using photochemical hydrosilylation that possessed substantial surface oxidation. In addition, photochemically modified SiNCs exhibit higher absolute photoluminescent quantum y...

Size dependence of the stability, electronic structure, and optical properties of silicon nanocrystals with various surface impurities

We present a comprehensive, ground-state density functional theory study of the size dependence of the optical and electronic properties and the stability of spherical silicon nanocrystals (NCs) with different impurities on the surface. We vary the size of the NCs from 1.0 to 3.5 nm, considering single-bonded $({\mathrm{CH}}_{3}$, F, Cl, OH) and double-bonded (O, S) impurities and bridged oxygen. We show that the density of states (DOS) and absorption indices of the NCs with single-bonded impurities are very similar to each other and the fully hydrogenated NCs, except for the 1.0-nm NCs, where a slight difference is present. In the case of the NCs with double-bonded impurities, the DOS and absorption indices exhibit a significant difference, compared to the fully hydrogenated NCs, for sizes up to 2.5 nm. We argue that this difference arises from the difference in the contribution from the impurity to the states around the gap, which can considerably change the character of the states. We demonstrate that the double-bonded impurities contribute significantly to the states around the gap, compared to the single-bonded impurities, causing changes in the symmetry of these states. This observation was further supported by analyzing the changes of the Fourier transform of the charge densities of the highest occupied and lowest unoccupied eigenstate. We also show that the formation energies of NCs with bridged oxygen and fluorine are the lowest, regardless of the size. Furthermore, we show that high hydrogen concentration can be used to suppress the addition of oxygen and fluorine on the surface of the Si NCs.

Electronic structure and optical properties of silicon nanocrystals

We have studied electronic structure and optical properties of Silicon nanocrystals by using density functional theory. The Silicon nanocrystals of diameters 1.0 nanometer up to 4.6 nanometer using the pseudopotential density functional theory were analysed. We have used real space three dimensional grid to approximate the real space integrals, charge densities and potentials, and numerical atomic orbitals allowing for very efficient reliable calculations. The fitness of the grid was governed by the plane wave cutoff. We have used plane wave cur off of 160Ry, which converged energies and density of state for bulk silicon. The basis size for silicon atoms was single with polarization orbitals and for hydrogen double. We have performed bench mark calculations for small silicon clusters. It was found that the cancellation of many body effects and the excitonic effects have given rise to density function theory the highest occupied molecular orbital-the lowest unoccupied molecular orbital gaps comparable with the experimental optical gaps. We have explored three types of structural models for nanocrystals, a spherical model and two polyhedral Wulff type structures and found them equivalent in terms of cohesive energies and density of states. We found that the narrowing of the valence band states contributed to the blue shift of the band gap, as a function of reduced size of the nanocrystals. We found good agreement in theory and experimental results.

Optical properties of silicon nanocrystals embedded in Si3N4 matrix measured by spectroscopic ellipsometry and UV-Vis-NIR spectroscopy

In this paper, we report a spectroscopic ellipsometry study of the optical properties of silicon nanocrystals (Si-ncs) embedded in silicon nitride matrix. The nanocomposite thin-films were elaborated by radiofrequency plasma enhanced chemical vapor deposition from ammonia and silane precursors, followed by high temperature annealing. Bruggeman effective medium approximation combined with the Tauc–Lorentz dispersion law was found to be an appropriate model in describing the ellipsometric data, and provided a fine determination of the dielectric functions or complex permittivity of Si-ncs embedded in silicon nitride. It is shown that the dielectric functions of Si-ncs undergo a large reduction in amplitude and broadening compared to the dielectric function of the bulk crystalline Si. Consequently to the disappearance of direct transition energy E1 and E2, the imaginary part ϵ2 of the dielectric function of Si-ncs exhibits a single line shape centered between E1 and E2. With decreasing Si-ncs size, we observe a red-shift of ϵ2 which cannot be attributed to bandgap expansion, but is better explained by electron–phonon interactions in the case of a Si3N4 matrix with high Young modulus. According to Tauc–Lorentz dispersion law, the obtained bandgap values of Si-ncs are between 1.58 eV and 1.67 eV for Si-ncs with diameters from 4.6 nm to 3.8 nm, which is in good agreement with measurements from UV-Vis-NIR spectroscopy.

(Invited) Transport and Electroluminescence Properties of Size-Controlled Silicon Nanocrystals Embedded in SiO2 Matrix Following the Superlattice Approach

We report a throrough analysis of the electrical and optical properties of silicon nanocrystals (Si-NCs)/SiO2 superlattices (SL), deposited on Si substrate by means of plasma-enhanced chemical-vapor deposition. A post-deposition annealing treatment was performed to induce phase separation and, therefore, precipitate the Si excess in the form of Si NCs. An excellent size control is reached by the limited thickness of the silicon rich layer of the SL. A discussion on the NC size effect on the injection and transport (electrical), and on absorption and electroluminiscence (electro-optical) is presented. An strong correlation between charge transport and electroluminescence is revealed. This work demonstrates that Si-NCs are a promising photovoltaic and luminescent material system; especially in the case of a SL-based material.

Size-dependent optical absorption of silicon nanocrystals embedded in SiO2/Si3N4 hybrid matrix

Optical constants of silicon quantum dots (Si QDs) in SiO2/Si3N4 hybrid matrix on quartz substrates have been determined in the wavelength range of 350–2500nm. Forouhi–Bloomer (F-B) dispersion model is applied to describe the optical properties of silicon nanocrystals and the mismatch due to global fitting is fixed point by point. To extract the specific optical bandgap of silicon nanocrystal samples, Tauc-plots with γ=1/2 and γ=1/3 are used and discussed for the spectral range of 2eV–3eV. The effect of quantum dot size on the optical bandgap is explored. The results from both Tauc-plot and photoluminescence show the trend that the optical bandgap increases with decreasing quantum dot size, which agrees with the prediction of quantum confinement.

Frontispiece: Size independent blue luminescence in nitrogen passivated silicon nanocrystals (Phys. Status Solidi A 10/2012)

Surface chemistry can play an important role in determining the optical properties of silicon nanocrystals. On this page, referring to the article by Mita Dasog and Jonathan G. C. Veinot (pp. 1844–1846) it is shown that by changing the surface of silicon nanocrystals, their emission color can be tuned. Hydride surface terminated silicon nanocrystals emit red/orange whereas, when surface groups attached through nitrogen atoms are present they emit blue. Chloride atoms quench the photoluminescence. The photoluminescence properties are shown as a background together with a transmission electron micrograph image of silicon nanocrystals.

Comparative study of the nonlinear optical properties of Si nanocrystals fabricated by e‐beam evaporation, PECVD or LPCVD

AbstractThe nonlinear optical properties of silicon nanocrystals (Si‐nc) embedded in oxide matrices have been studied in samples obtained by three different technological process: e‐beam evaporation, plasma enhanced or low pressure chemical vapor deposition (PECVD and LPCVD, respectively). Z ‐scan measurements were performed in all the samples at 1064 nm by using ns‐pulses of a Nd:YAG laser. Similar nonlinear refractive index was found in the three systems, while differences in the nonlinear absorption were found as a function of the deposition method.The nonlinear results have been evaluated in terms of the crystalline and amorphous fractions from Raman scattering and photoluminescence (PL) measurements. While PL measurements show a emission spectrum similar for all the samples, Raman spectra reveal a sizeably different Si crystalline/amorphous ratio, depending on the deposition method. Therefore, the interface between the SiO2 matrix and Si‐nc plays a crucial role in determining the nonlinear optical response of the Si‐nc rich layers. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

On the nature of the stretched exponential photoluminescence decay for silicon nanocrystals

The influence of hydrogen rate on optical properties of silicon nanocrystals deposited by sputtering method was studied by means of time-resolved photoluminescence spectroscopy as well as transmission and reflection measurements. It was found that photoluminescence decay is strongly non-single exponential and can be described by the stretched exponential function. It was also shown that effective decay rate probability density function may be recovered by means of Stehfest algorithm. Moreover, it was proposed that the observed broadening of obtained decay rate distributions reflects the disorder in the samples.

Structure and Optical Properties of Silicon Nanocrystals Embedded in Amorphous Silicon Thin Films Obtained by PECVD

Silicon nanocrystals embedded in amorphous silicon matrix were obtained by plasma enhanced chemical vapor deposition using dichlorosilane as silicon precursor. The RF power and dichlorosilane to hydrogen flow rate ratio were varied to obtain different crystalline fractions and average sizes of silicon nanocrystals. High-resolution transmission electron microscopy images and RAMAN measurements confirmed the existence of nanocrystals embedded in the amorphous matrix with average sizes between 2 and 6 nm. Different crystalline fractions (from 12% to 54%) can be achieved in these films by regulating the selected growth parameters. The global optical constants of the films were obtained by UV-visible transmittance measurements. Effective band gap variations from 1.78 to 2.3 eV were confirmed by Tauc plot method. Absorption coefficients higher than standard amorphous silicon were obtained in these thin films for specific growth parameters. The relationship between the optical properties is discussed in terms of the different internal nanostructures of the samples.

Critical Role of Dopant Location for P-Doped Si Nanocrystals

The doping of phosphorus (P) provides an additional means to control the optical properties of silicon nanocrystals (Si NCs). The P-doping-induced changes in the optical properties of Si NCs, however, have not been consistently understood. On the basis of first-principles calculations, we explain the P-doping-induced infrared absorption of Si NCs and the effect of P-doping on the light emission from Si NCs. The explanations are enabled by the investigation of the locations of P in Si NCs, including a variety of locations at the surface of Si NCs. We show that the light emission from Si NCs critically depends on the location of P. Transitions involving P-doping-induced defect energy levels lead to the infrared absorption of Si NCs.