Imbedding germanium quantum dots in silica by a modified Stöber method
Introduction
Solar energy represents a potential alternative clean energy generation source. Unfortunately, it is presently unable to compete with traditional hydrocarbon combustion in terms of cost per kW/hour. If solar is to compete (in the absence of government subsidies) it must either have a lower cost or higher efficiencies than those currently available. Efforts are ongoing to create low cost solar cells [1], [2], [3], [4]. On the other hand one method of improving solar cell efficiency is through the creation of tandem solar cells, which have multiple p-n junctions allowing the absorption of many wavelengths of light [5], [6]. The majority of tandem cell designs are based upon III-V semiconductor cells [7]; however, Green and co-workers proposed a unique approach to the inclusion of a tandem layer into first generation silicon solar technology [8], [9], [10], by the inclusion of an array of semiconductor quantum dots coated within an insulating material. The major drawback with this structure is that fabrication on a large scale is difficult, in particular the control over the QD size and the QD…QD distance [8], [9], [10]. In seeking an alternative approach, we have recently proposed that if a suitable silicon or germanium QD were coated with a uniform coating of silica (i.e., Si@SiO2 or Ge@SiO2), then arraying the resulting spheres would result in a QD…QD distance defined by the coating thickness (Fig. 1) [11].
Our initial attempts involved the coating of hydrophilic Si or Ge QDs with silica using liquid phase deposition (LPD). Unfortunately, while some silica particles were prepared with a single QD inside (e.g., Ge@SiO2), others showed evidence for multiple QDs per silica sphere, i.e., Gex@SiO2 [6]. Furthermore, while films of both Si@SiO2 and Ge@SiO2 showed good photocurrent indicating a sufficient number of QD…QD distances were within the 10 nm required for electron transfer to be possible [8], [9], [10], transmission electron microscopy (TEM) analysis suggested that these were not a majority. A second issue involved the hydrofluoric acid waste produced during the LPD process using hexafluorosilicic acid as the silica precursor [12]. What is needed is a method that can reproducibly produce solely individual Si or Ge QDs within single silica particles of a suitable diameter such that thin films will maximize the fraction of QD…QD distances that are within the 10 nm limit.
In 1968, Werner Stöber published a method of creating spherical, mono-dispersed silica nanoparticles ranging in size from 50 nm to 2 μm [13]. Although mono-dispersed particles are produced using this original method, they cannot be synthesized with small enough diameters for our present application. However, Yokoi et al. have reported that by catalyzing the hydrolysis reaction used to make the silica particles with a bulkier organic base such as l-lysine, mono-dispersed silica nanoparticles as small as 12 nm have been reported [14], [15]. Based upon this result, we propose that germanium QDs can be coated with a thin layer of silica by using the modified Stöber method with a bulky base and using the germanium quantum dots as seeds. The results of this study are reported herein.
Section snippets
Materials and methods
Tetraoctylammonium bromide (98%), lithium aluminum hydride (1.0 M in tetrahydrofuran), chloroplatinic acid hydrate (99.9%), allylamine (99+%), and l-lysine (97%) were obtained from Aldrich Chemical Company. Germanium tetrachloride (99.99%) was obtained from Acros Organics. Tetraethoxysilane (TEOS) (99.999+%) was obtained from Alfa Aesar. Ethanol (200 proof) was obtained from Decon Laboratories. Toluene (99.98%) was obtained from EMD, and was distilled under argon prior to use. Methanol (≥99.8%)
Results and discussion
While the original design of a tandem cell layer by Green and co-workers used Si QDs within a silica matrix due to the method of manufacture [8], [9], [10], Ge QDs are ideal for laboratory experiments because they can be easily produced with inexpensive, clean materials, and their quantum confinement is relatively large (11.5 nm), thus making their production easier than quantum dots with smaller Bohr radii [17]. They also are distinguishable from silica in transmission electron microscope (TEM)
Conclusions
Germanium QDs were successfully coated with a thin layer of silica using a modified Stöber process with l-lysine. The unmodified process was unsuccessful, indicating that the bulkier base made it possible for the quantum dots to seed the synthesis of the particles. In comparison with our previously reported LPD method, the modified Stöber process produces a more uniform material in which each QD acts as a seed to a silica particle. Most importantly, in thin films of the Ge@SiO2 particles the
Acknowledgments
This work was supported by Natcore Technology, Inc. and the Robert A. Welch Foundation (C-0002). One of the authors (A.R.B.) is the scientific founder of Natcore Technology, Inc. and has an equity interest in the company.
References (19)
- et al.
Thin Solid Films
(2011) - et al.
Solar Energy Materials and Solar Cells
(2011) - et al.
Thin Solid Films
(2006) - et al.
Materials Science in Semiconductor Processing
(2012) - et al.
Journal of Colloid and Interface Science
(1968) Journal of Colloid and Interface Sciences
(2007)- et al.
Advanced Materials
(1991) - et al.
Progress in Photovoltaics
(2002) - et al.
Journal of Applied Physics
(1961)
Cited by (8)
Facile, single-pot preparation of nanoporous SiO<inf>2</inf> particles (carrier) with AgNPs at core and crust for controlled disinfectant release
2019, Journal of Saudi Chemical SocietyCitation Excerpt :SiO2 nanoparticles made using the Stöber method are shown in Fig. 2(a). Based on previous reports [22,23], clear SiO2 nanoparticles of size 150 nm to 250 nm can be seen. Fig. 2(b) shows an HRTEM image of SiNP-AgCC produced by the modified Stöber method; notably, AgNPs can be seen on the surface and inside the SiO2 particle.
Fast responsive thermally stable silica microspheres for sensing evaluation: sol–gel approach
2020, Journal of Sol-Gel Science and TechnologyElectronic and spectroscopic properties of Ge nanocrystals using diamondoid structures: A density functional theory study
2016, International Journal of Modern Physics B