Nanocrystalline Silicon Quantum Dots Research Articles

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 Eg(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.

High-efficiency silicon hybrid solar cells employing nanocrystalline Si quantum dots and Si nanotips for energy management

We have achieved an efficiency of 13.73% in PEDOT:PSS/Si hybrid solar cells by employing nanocrystalline silicon quantum dots (nc-Si QDs). This form of efficiency enhancement derives from energy transfer from nc-Si QDs to the underlying layer of the Si nanostructure with high carrier separation and propagation via the core-shell of Si/PEDOT:PSS layer, leading to enhanced photovoltaic effects. The short-circuit current density, open circuit voltage and fill factor are increased from 34.07mA/cm2 to 37.85mA/cm2, 0.539V to 0.595V, and 56.51% to 60.91%, respectively. Higher short-circuit density and open-circuit voltage are obtained due to energy transfer management and optimization of surface defects. These techniques hold future promise for developing energy transfer-managed, low-cost and high-efficiency photovoltaic cells.

Electron transport through a single nanocrystalline silicon quantum dot between nanogap electrodes

We experimentally study the electron transport through a single nanocrystalline silicon quantum dot between the nanogap electrodes. We fabricate the device by depositing a nanocrystal into an ∼10-nm gap using a very high frequency plasma cell. The Coulomb diamond size depends on the number of electrons at 4.5 K, which indicates that even–odd shell filling occurs. The charging energy is estimated to be ∼11 meV, which is consistent with the size of the silicon nanocrystal. The perpendicular magnetic field dependence of the Coulomb diamonds demonstrates the Zeeman splitting as well as orbital energy evolution.

Opto-electronic properties of P-doped nc-Si–QD/a-SiC:H thin films as foundation layer for all-Si solar cells in superstrate configuration

With the advent of nc-Si solar cells having improved stability, the efficient growth of nc-Si i-layer of the top cell of an efficient all-Si solar cell in the superstrate configuration prefers nc-Si n-layer as its substrate. Accordingly, a wide band gap and high conducting nc-Si alloy material is a basic requirement at the n-layer. Present investigation deals with the development of phosphorous doped n-type nanocrystalline silicon quantum dots embedded in hydrogenated amorphous silicon carbide (nc-Si–QD/a-SiC:H) hetero-structure films, wherein the optical band gap can be widened by the presence of Si–C bonds in the amorphous matrix and the embedded high density tiny nc-Si–QDs could provide high electrical conductivity, particularly in P-doped condition. The nc-Si–QDs simultaneously facilitate further widening of the optical band gap by virtue of the associated quantum confinement effect. A complete investigation has been made on the electrical transport phenomena involving charge transfer by tunneling and thermionic emission prevailing in n-type nc-Si–QD/a-SiC:H thin films. Their correlation with different phases of the specific heterostructure has been carried out for detailed understanding of the material, in order to improve its device applicability. The n-type nc-Si–QD/a-SiC:H films exhibit a thermally activated electrical transport above room temperature and multi-phonon hopping (MPH) below room temperature, involving defects in the amorphous phase and the grain-boundary region. The n-type nc-Si–QD/a-SiC:H films grown at ∼300 °C, demonstrating wide optical gap ∼1.86–1.96 eV and corresponding high electrical conductivity ∼4.5 × 10−1–1.4 × 10−2 S cm−1, deserve to be an effective foundation layer for the top nc-Si sub-cell of all-Si solar cells in n-i-p structure with superstrate configuration.

A Quantum Confinement Study of the Electronic Energy of some Nanocrystalline Silicon Quantum-Dots

We have employed a quantum confinement (QC) model to the study of different shapes of nanocrystalline silicon (nc-Si) quantum dot. Each dots (shapes), although within the limits of an effective diameter of 3nm, exhibits divergence leading to different electronic energy based on the transitions from the quantum selection rule. Also, the graphical representation of the energies from each shape as a function of the effective diameter gives a qualitatively similar spectrum of discrete energies. The results obtained in this work using QC model are in good agreement with experiment and other models in literature.

A Quantum Confinement Study of the Electronic Energy of some Nanocrystalline Silicon Quantum-Dots

We have employed a quantum confinement (QC) model to the study of different shapes of nanocrystalline silicon (nc-Si) quantum dot. Each dots (shapes), although within the limits of an effective diameter of 3nm, exhibits divergence leading to different electronic energy based on the transitions from the quantum selection rule. Also, the graphical representation of the energies from each shape as a function of the effective diameter gives a qualitatively similar spectrum of discrete energies. The results obtained in this work using QC model are in good agreement with experiment and other models in literature.

Formation of three-dimensionally integrated nanocrystalline silicon particles by dip-coating method

Printable technologies using silicon nanoink, in which nanocrystalline silicon (nc-Si) quantum dots are dispersed in solvents, are promising for novel electron and photonic device applications. The dip-coating method is applied for the first time to fabricate three-dimensionally integrated structures of nc-Si quantum dots with a uniform size of 10 nm prepared by the very high frequency plasma decomposition of silane gas. We have clarified the major problem of the dip-coating method, which is the formation of stripe structures. To circumvent this problem, we have proposed two methods: coating onto line-and-space-patterned substrates and utilization of electrophoresis force. We have successfully demonstrated the control of the position and number of layers of nc-Si by using a line-and-space-patterned substrate, however, with a limited shape. We have clarified the conditions of the formation of stripe-free regions by varying applied voltage and nc-Si concentration in the electrophoresis method.

Enhancing Quantum Dots for Bioimaging using Advanced Surface Chemistry and Advanced Optical Microscopy: Application to Silicon Quantum Dots (SiQDs).

Fluorescence lifetime imaging microscopy is successfully demonstrated in both one- and two-photon cases with surface modified, nanocrystalline silicon quantum dots in the context of bioimaging. The technique is further demonstrated in combination with Förster resonance energy transfer studies where the color of the nanoparticles is tuned by using organic dye acceptors directly conjugated onto the nanoparticle surface.

Nanocrystalline Si/SiO2 core-shell network with intense white light emission fabricated by hot-wire chemical vapor deposition

We report the fabrication of a stable Si/SiO2 core-shell network using hot-wire chemical vapor deposition on a silicon substrate at a relatively low substrate temperature of 200 °C. Structural investigations using transmission electron microscopy and X-ray diffraction confirm the presence of nanocrystalline silicon and silicon dioxide quantum dots in the form of a core-shell network embedded in the amorphous SiOx matrix, while selected area electron diffraction confirms the formation of a core-shell structure. The core-shell structure exhibits a bright white emission that can be seen with the unaided eye at room temperature without any post-annealing treatments, and the observed photoemission does not alter in color or intensity after prolonged laser exposure. Additional measurements are performed while varying the laser power and optical gain is found in the as-deposited material. Intense stable white luminescence is observed and shows the prospective for various optical and biological applications in the future.

Self-assembled nc-Si/a-SiNx: H quantum dots thin films: An alternative solid-state light emitting material

Nanocrystalline silicon quantum dots (QDs) of varying size from ~5.4 to 2.2nm embedded in amorphous silicon-nitride matrix (nc-Si-QDs/a-SiNx:H) were prepared via ICP-CVD (13.56MHz), using a mixture of silane (SiH4), ammonia (NH3) gases in H2-diluted plasma, by changing the low deposition pressure in a narrow range, 10–50mTorr at low substrate temperature (250°C). Strong visible photoluminescence (PL) tunable over a significantly wide range (1.67–3.02eV) has been observed. The origin of the PL is mostly attributed to band-to-band recombination due to the quantum confinement effect (QCE) in the nanocrystalline silicon QDs. A large amount of atomic hydrogen flux that originates due to the high degree dissociation of the gas molecules in high-density inductively coupled plasma (ICP) passivates well the nonradiative dangling bonds and helps in growing plenty of ultra-nanocrystallites that demonstrate intense visible photoluminescence. The red-shift of Raman peak and the corresponding line broadening have been associated to the confinement of optical phonons within the nc-Si QDs. The widening of band gap and tune-ability of visible photoluminescence over a notably wide range along with significantly high electrical conductivity of the material demonstrates enormous promise for its utilization in the fabrication of effective solid-state light emitting devices.

Tunable photoluminescence from nc-Si/a-SiNx:H quantum dot thin films prepared by ICP-CVD

Intense visible photoluminescence (PL) tunable within 1.66-2.47 eV, under UV 325 nm excitation, was obtained from nanocrystalline silicon quantum dots (∼5.72-1.67 nm in diameter) embedded in amorphous silicon-nitride matrix (nc-Si/a-SiN(x):H) prepared in RF-ICPCVD (13.56 MHz) at substrate temperatures between 400 to 150 °C. The dominant component of PL, having a narrow band width of ∼0.16-0.45 eV, originates from quasi-direct band-to-band recombination due to quantum confinement effect (QCE) in the nanocrystalline silicon quantum dots (nc-Si QDs) of appropriate size; however, the contribution of defects arose at lower substrate temperatures leading to asymmetric broadening. Intense atomic hydrogen flux in high-density inductively coupled plasmas (ICPs) provides a very high surface coverage, passivates well the nonradiative dangling bonds, and thereby favors the PL intensity. The average size of nc-Si QDs measured by HR-TEM appears consistent with similar estimates from Raman studies. The red shift of the Raman line and corresponding line broadening originates from the confinement of optical phonons within nc-Si QDs. Photoluminescence emerging from nc-Si/a-SiN(x):H quantum dots obtained from the low temperature and single-step plasma processing holds great promise for the fabrication of light-emitting devices and flexible flat panel displays.

SiOx nanowires with intrinsic nC-Si quantum dots: the enhancement of the optical absorption and photoluminescence

We report a very simple method for the fabrication of amorphous silicon rich oxide nanowires (a-SiOx NWs) with intrinsic nanocrystalline silicon quantum dots (nC-Si QDs) with a high density and controlled size, by thermally annealing thin films of nC-Si quantum dots embedded in the a-SiOx matrix, overcoated by an ultra-thin Au catalyst layer. The size and density of the Si-QDs in the basic matrix are controllable by changing the plasma parameters during the growth of the nC-Si/a-SiOx thin films. The diameter of the grown SiOx nanowires is controlled by merely varying the thickness of the Au-coating. The formation of the 1D NWs with rigid boundaries influences a shrinkage in the size of the intrinsic 0D QDs; the size-reduction of the QDs is more prominent in narrower NWs obtained from thinner catalyst layers. This unique quantum-dot/nanowire composite system demonstrates a significantly improved optical absorption and reduced reflection in the entire UV-visible range of the solar spectrum compared to its thin film structure, and those are again pronounced by reducing the dimension of the nanowires. The photoluminescence properties of this composite system demonstrate a strong room temperature emission band in the range of 400–600 nm with a peak at ∼519 nm. The PL peak of the NW film undergoes a large blue shift by 150 nm from that of the annealed QD films which has been attributed to the reduction in the average size of the intrinsic nC-Si QDs during the growth of the a-SiOx NWs. These novel a-SiOx NWs with intrinsic nC-Si QDs, produced by a simple synthesis technique involving a solid–liquid–solid (SLS) growth process, deserve enough promise in the field of photovoltaics and light emitting devices.

Photoluminescence of Nanocrystalline Silicon Quantum Dots with Various Sizes and Various Phosphorus Doping Concentrations Prepared by Very High Frequency Plasma

Photoluminescence of Nanocrystalline Silicon Quantum Dots with Various Sizes and Various Phosphorus Doping Concentrations Prepared by Very High Frequency Plasma

Photoluminescence of Nanocrystalline Silicon Quantum Dots with Various Sizes and Various Phosphorus Doping Concentrations Prepared by Very High Frequency Plasma

Nanocrystalline silicon quantum dots (nc-Si QDs) with various sizes and various doping concentrations were fabricated by supplying different flow rates of silane (SiH4) gas and phosphine (PH3) gas, respectively, under very high frequency (VHF; 144 MHz) plasma. With an increase in the flow rate of SiH4 gas, the mean size of nc-Si QDs decreases, and photoluminescence (PL) measurements and time-resolved photoluminescence (TRPL) measurements confirm that the quantum confinement becomes weaker. For a small amount of doping PH3 gas, termination of dangling bonds at the interface between the crystal core and the oxide shell in the nc-Si QDs could be supposed on the basis of PL and TRPL measurements. In contrast, for a large amount of doping PH3 gas, Auger recombination could dominate the luminescence properties of nc-Si QDs, and it could decrease the PL intensity and shorten the TRPL decay lifetime.

Nanocrystalline silicon and silicon quantum dots formation within amorphous silicon carbide by plasma enhanced chemical vapour deposition method controlling the Argon dilution of the process gases

Structural and optical properties of the amorphous silicon carbide (a-SiC:H) thin films deposited by radio frequency plasma enhanced chemical vapour deposition method from a mixture of silane (SiH4) and methane (CH4) diluted in argon (Ar) have been studied with variation of Ar dilution from 94% to 98.4%. It is observed that nanocrystalline silicon starts to form within the a-SiC:H matrix by increasing the dilution to 96%. With further increase in Ar dilution to 98% formation of the silicon nanocrystals (nc-Si) with variable size is enhanced. The optical band gap (Eg) of the a-SiC:H film decreases from 2.0eV to 1.9eV with increase in Ar dilution from 96% to 98% as the a-SiC:H films gradually become Si rich. On increasing the Ar dilution further to 98.4% leads to the appearance of crystalline silicon quantum dots (c-Si q-dots) of nearly uniform size of 3.5nm. The quantum confinement effect is apparent from the sharp increase in the Eg value to 2.6eV. The phase transformation phenomenon from nc-Si within the a-SiC:H films to Si q-dot were further studied by high resolution transmission electron microscopy and the grazing angle X-ray diffraction spectra. A relaxation in the lattice strain has been observed with the formation of Si q-dots.

Fabrication of Nanosilicon Ink and Two-Dimensional Array of Nanocrystalline Silicon Quantum Dots

An assembly of nanoparticles using a colloidal solution is promising for the fabrication of future highly integrated electron and photoelectronic devices because of low manufacturing cost, flexible substrates, and alternative methods that can overcome the limitation of top-down technology. We have successfully prepared two-dimensional arrays of nanocrystalline silicon (nc-Si) quantum dots with a uniform size of 10 nm. However, the area of two-dimensional arrays has been limited because of the problems of dissolution in water and agglomeration of nc-Si due to a high surface reactivity. The key issue is the surface modification of nc-Si particles. In this study, we have demonstrated the evaluation of surface modification states of nc-Si QDs by zeta potential and particle size distribution measurements. As a result of the optimization of the surface modification process, we have successfully obtained a well-dispersed nc-Si QD solution, namely, nanosilicon ink. Furthermore, we have successfully fabricated a two-dimensional array of nc-Si QDs using the Langmuir-Blodgett film method in the entire 1 ×1 cm2 silicon substrate.

Integration of Tunnel-Coupled Double Nanocrystalline Silicon Quantum Dots with a Multiple-Gate Single-Electron Transistor

We report on integration of double nanocrystalline silicon quantum dots (nc-Si QDs) of approximately 10 nm in diameter onto the multiple-gate single-electron transistor (SET) used as a highly-sensitive charge polarization detector. The SET with a single charging island is first patterned lithographically on silicon-on-insulator, and the multiple-gate bias dependence of the Coulomb current oscillation is characterized at 4.2 K. The coupling capacitance parameters between the SET charging island and the multiple-gate are estimated and compared with those obtained by using the three-dimensional capacitance simulation. Double nc-Si QDs are then deposited in the immediate vicinity of the charging island of the SET by using the very-high frequency plasma deposition technique. We perform the single-electron circuit simulations and demonstrate that only ±e charge polarization of the double QDs can be sensed as a shift of the Coulomb oscillation peaks.

Three-Dimensional Numerical Analysis of Switching Properties of High-Speed and Nonvolatile Nanoelectromechanical Memory

Static and dynamic mechanical properties of the movable floating gate are investigated for a newly proposed highspeed and nonvolatile nanoelectromechanical memory, which features a buckled floating gate incorporating the nanocrystalline silicon quantum dots integrated onto the gate of a MOSFET. By conducting a 3D finite element simulation, we analyze the structural parameter dependence of the switching force F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> needed for the buckled floating gate to flip-flop between its bistable states and derive the relationship F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> infin L <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> T Z <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> where L, T, and Z <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> represent the length, thickness, and equilibrium displacement of the buckled floating gate, respectively. We demonstrate that the switching frequency can be increased while maintaining the switching force when we downscale all the floating gate dimensions proportionally along with the scaling law. We also show that the switching voltage can be reduced down to less than 15 V while maintaining the ON/OFF operation range of the sense MOSFET by optimizing the cavity structure which sustains the inside buckled floating gate

Observation and Analysis of Tunneling Properties of Single Spherical Nanocrystalline Silicon Quantum Dot

Electron tunneling is investigated experimentally and theoretically for a single nanocrystalline silicon quantum dot with a diameter of less than 10 nm. Nanocrystalline silicon quantum dots are deposited by very high frequency (VHF) plasma chemical vapor deposition (CVD) sparsely over the oxide-covered silicon substrate. The dots are then naturally oxidized at room temperature by exposing it to ambient air, resulting in an ultrathin oxide tunnel barrier. A current peak is clearly observed at room temperature with the highest peak-to-valley current ratio of about 17 by contact-mode atomic force microscopy (AFM). We also perform three-dimensional numerical simulations of the transmission spectrum and tunneling current based on the scattering matrix theory. In the current–voltage (I–V) characteristics calculated at room temperature we demonstrate the resonant tunneling current peaks associated with the quasi-bound state formed in a spherical silicon quantum dot.

Electron localization and resonant tunneling in uniform nanocrystalline silicon quantum dot systems

In this paper, we review our recent investigation of the electronic transport in hydrogenated nanocrystalline silicon thin film grown on crystalline silicon substrate. The variable magnetic-field Hall measurements have been applied to extract the separated carrier information about the nc-Si:H, c-Si and their interface, where the formation of two-dimensional electron gas is clearly observed. With a decrease in the temperature, the nc-Si:H thin film shows a transition from the extended state to the band-tail state, together with a positive magneto-conductivity at extremely low temperatures. These results reveal the good uniformity of the quantum dots in the nc-Si:H thin film and the electron localization. Furthermore, we have successfully observed clear step-like and spike-like structures for the electrons resonant tunneling through the quasi-2D and 0D states in the I– V curves of the nc-Si:H/ c-Si diode under strong electric field at low temperatures.