- Functionalization and characterization of Si quantum dots (Si-QDs)
- Evaporation and deposition of Si-QDs in ultra-high-vacuum (UHV) at relatively low temperatures
- Biological and medical applications
- Applications in floating gate memory devices
- Desperately seeking silicon: A novel thermoelectric material
- Design and synthesis of noble metal nanoparticles embedded in porous Si - A novel approach to enhance transfer efficiency in redox reactions
- Highly photoluminescent alkylated silicon quantum dots for photovoltaic applications
Functionalization and characterization of Si quantum dots (Si-QDs)
Si-QDs were prepared by electrochemical method. Photoluminescent porous silicon layers were formed by galvanostatic anodization of a boron-doped p-Si (100) oriented wafer in a 1:1 v/v solution of 48% aqueous HF and EtOH solution. The electrochemical cell was machined from PTFE and circular in cross-section (1 cm diameter). The silicon wafer was sealed to the base using a VitonTM O-ring. The counter electrode was a piece of tungsten wire coiled into a loop to improve the uniformity of the current distribution and the etching was carried out using a constant current source (KEITHLEY 2601). A layer of luminescent porous silicon was made at high current density (5 min at 350 mA cm-2). The solution was decanted and the fluorescing porous silicon chip was dried under the vacuum of a rotary pump. The dry, hydrogen-terminated porous silicon chip was then transferred into a Schlenk flask on a greasefree vacuum line (employing Young’s taps) and refluxed overnight in a dry toluene solution containing 1 M of the alkene (1-hexene, 1-octene, 1-undecene or 1,9-decadiene, Merck). Undissolved silicon particles were filtered off before all solvent and unreacted alkene was removed under reduced pressure with a rotary pump and the powder was redissolved by stirring in toluene, dichloromethane (DCM) or trichloromethane (TCM). In our method hydrogenterminated porous silicon films refluxed in toluene or mesitylene solutions of alk1enes break into small particles whilst the alkene undergoes hydrosilation at the particle surface. These particles are strongly luminescent and very stable towards oxidation under ambient conditions.
References
- Chao, Y., Krishnamurthy, S., Montalti, M., Lie, L. H., Houlton, A., Horrocks, B. R., Kjeldgaard, L., Dhanak, V. R., Hunt, M. R. C., and Šiller, L. Reactions and luminescence in passivated Si nanocrystals induced by vacuum ultraviolet and soft-x-ray photons. J. Appl. Phys. 98, 044316 (2005).
- Chao Y, Houlton A, Horrocks BR, Hunt MRC, Poolton NRJ, Yang J and Šiller, L. Optical luminescence from alkyl-passivated Si nanocrystals under vacuum ultraviolet excitation: Origin and temperature dependence of the blue and orange emissions. Appl. Phys. Letts. 88, 263119 (2006).
- Lie, L. H., Duerdin, M., Tuite, E. M., Houlton, A., and Horrocks, B. R. Preparation and characterisation of luminescent alkylated silicon quantum dots. J. Electroanal. Chem. 538, 183-190 (2002).
- Lie, L. H., Patole, S. N., Pike, A. R., Ryder, L. C., Connolly, B. A.,Ward, A. D., Tuite, E. M., Houlton, A., and Horrocks, B. R. Immobilisation and synthesis of DNA on Si(111), nanocrystalline porous silicon and silicon nanoparticles. Faraday Discuss. 125, 235-249 (2004).
Evaporation and deposition of Si-QDs in ultra-high-vacuum (UHV) at relatively low temperatures
This is an extremely novel and unexpected observation: quantum dots can be evaporated into vapour in UHV and then deposited onto a cold substrate. The intact sublimation is enabled by a combination of anomalously weak inter-particle interactions with the extremely high thermal stability of the alkyl-coated silicon nanoparticles. This work has the potential to launch a new direction in research on thermally stable nanomaterials that can be processed by sublimation in UHV.
References
- Yimin Chao, Lidija Šiller, Satheesh Krishnamurthy, Paul R. Coxon, Ursel Bangert, Mhairi Gass, Lisbeth Kjeldgaard, Samson N. Patole, Lars H. Lie, Norah O’Farrell, Thomas A. Alsop, Andrew Houlton and Benjamin R. Horrocks, Evaporation and deposition of undecyl-capped silicon nanocrystals in ultrahigh vacuum, Nature Nanotechnology, in press.
Biological and medical applications
Size selected silicon quantum dots can act as both luminescence probes (for spatial localisation) and Raman probes (to provide time-resolved biochemical information) inside living cells. With the observation of different signals between cancer cells and normal cells this method could have potential application in diagnosis.
The ideal memory devices should have such characteristics as: high memory performance with fast program/erase cycle speed, low operating power with high drive current, long memory retention time, high reliability and long lifetime. In current floating gate memories, charge is stored on a silicon floating gate that is separated from the substrate by a dielectric tunnelling barrier. Typically, the tunnelling barrier consists of a thin thermally-grown SiO2 layer. Silicon quantum dots memories, in which the SiO2 layer is replaced by a Si-QDs monolayer, have attracted considerable attention in the last few years as one of the simplest evolutions of the standard flash technology allowing for improved reliability and scaling perspectives.
Desperately seeking silicon: A novel thermoelectric material
Energy shortage and climate changes are two of the most serious problems threatening our society. What can nanosciences do to solve these problems with cost-effective and sustainable solutions? A thin film of functionalized silicon nanocrystals could be a potential climate saver.
Currently approximately 90 percent of power is generated by heat engines that use fossil fuel combustion as a heat source and typically operate at 30-40 percent efficiency. As a result, roughly 15 terawatts of heat is lost to the environment per year [1]. Thermoelectric materials convert heat into electric current. If they could be made more efficient thermoelectric modules could potentially convert part of this low-grade waste heat to electricity, or act as an alternative to photovoltaic cells for converting solar radiation into electricity. Unfortunately, current thermoelectric materials are very expensive and are nowhere near efficient enough to be used commercially. The efficiency and energy density of a material for thermoelectric applications is determined by the dimensionless figure of merit ZT:
where T is the absolute temperature, S is the Seebeck coefficient, σ is the electrical conductivity, and Κ is the thermal conductivity (Κe is the electronic contribution to the thermal conductivity, ΚL is the lattice thermal conductivity). Therefore, a good thermoelectric material should possess the following properties: low parasitic thermal conduction by electrons and phonons to retain the heat at the junction and to reduce the heat transfer losses; high Seebeck coefficient S = ∆V/∆T; little Joule heating (high electric conductivity σ).
[1] A. I. Hochbaum, R. K. Chen, R. D. Delgado, W. J. Liang, E. C. Garnett, M. Najarian, A. Majumdar, and P. D. Yang, Nature 451, 163 (2008).
[2] C. B. Vining, Nature 451, 255 (2008).
Design and synthesis of noble metal nanoparticles embedded in porous Si - A novel approach to enhance transfer efficiency in redox reactions
Selective oxidation and hydrogenation is a key technology in chemical industry, highly active and stable catalyst is desired to enhance transfer efficiency and reduce by-products, that is a way to enhance energy efficiency and reduce CO2 production. The features of catalyst supports are usually the crucial factors to improve the performance of a catalyst by sufficiently exerting the superiority of the active species. An ideal support should possess, at least, two essential functions: (1) maintaining high dispersion and stability of active components, and (2) facilitating the diffusion of reactants and the accessibility to active sites. Therefore, much effort has been made to create new catalyst supports that can provide an anti-sintering property and increase the accessibility to active species.1
Porous Si (pSi) possesses nanoporous holes in its microstructure rendering a large surface to volume ratio. pSi can be fabricated by anodization in which platinum cathode and silicon wafer anode immersed in Hydrogen Fluoride (HF) electrolyte. Corrosion of the anode is produced by running electrical current through the cell. It is noted that the running of constant DC is usually implemented to ensure steady tip-concentration of HF resulting in a more homogeneous porosity layer. After drying and surface modification to improve the stability, pSi is an ideal candidate to assemble noble metal catalyst. 2
Noble metal nanoparticles can be deposited onto pSi and form an interesting “rocks-in-forest” structure, which is envisaged to exhibit excellent activity/selectivity/stability in the selective oxidation (Ag/Si) or hydrogenation (Pt/Si, Pd/Si) process.
This project is to assemble a unique “rock-in-forest” structure with pSi and selected noble metal nanoparticles, through optimised etching parameters for pSi fabrication, investigating the effect of surface modification on stability of pSi and assembled structure, towards high conversion rate and selectivity for conversion of alcohol to its corresponding aldehyde or selective hydrogenation of citral.3
Figure 1: SEM image on cross section of porous silicon (left), and “rocks-in forest” structure (right)
References:
1. C. Zhang, P. Chen, J. Liu, Y. Zhang, W. Shen, H. Xu and Y. Tang, Chem. Commun., 2008, 3290.
2. P. R. Coxon, Q. Wang and Y. Chao, J. Phys. D: Appl. Phys., 2011, 44, in press.
3. G. Guo, F. Qin, D. Yang, C. Wang, H. Xu and S. Yang, Chem. Mater., 2008, 20, 2291.
Highly photoluminescent alkylated silicon quantum dots for photovoltaic applications
With a thin layer of Si-QDs on the top surface of a commercially available solar cell, the reflectivity of the surface is reduced. Hence, the light absorption will be increased while the reflection is decreased. The deposited Si-QDs can absorb short wavelength UV and emit visible light, which can be absorbed by the solar cell. Therefore, the efficiency can be improved.1, 2 The nature of photovoltaic application requires the Si-QDs should be hydrophobic. Thus, alkyl ligands are potential candidates.
The resulting Si-QDs were applied on the front surface of a conventional silicon solar cell. The surface morphology was examined with SEM and the total light absorption was measured by an integrating sphere set up. The overall efficiency was enhanced by 10%.
Fig. 1 Schematic mechanism of improving efficiency: (1) incident light; (2) transmitted to solar cell; (3) reflected light; (4) absorbed by Si-QDs; (5) emitted light from Si-QDs
1. G.-r. Chang, F. Ma, D.-y. Ma and K.-w. Xu, Nanotechnology, 2010, 21, 465605.
2. W. Ding, R. Jia, D. Wu, C. Chen, H. Li, X. Liu and T. Ye, J. Appl. Phys., 2011, 109, 054312.
