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Silicon quantum dots: surface matters

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Journal of Physics: Condensed Matter J. Phys.: Condens. Matter 26 (2014) 173201 (28pp)


Topical Review

Silicon quantum dots: surface matters K Dohnalová1, T Gregorkiewicz1 and K Kůsová2 1

  Van der Waals-Zeeman Institute, University of Amsterdam, Science Park 904, NL-1098 XH Amsterdam, The Netherlands 2   Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10, 162 00, Prague 6, Czech Republic E-mail: [email protected] Received 30 November 2013, revised 6 February 2014 Accepted for publication 7 February 2014 Published 8 April 2014 Abstract

Silicon quantum dots (SiQDs) hold great promise for many future technologies. Silicon is already at the core of photovoltaics and microelectronics, and SiQDs are capable of efficient light emission and amplification. This is crucial for the development of the next technological frontiers—silicon photonics and optoelectronics. Unlike any other quantum dots (QDs), SiQDs are made of nontoxic and abundant material, offering one of the spectrally broadest emission tunabilities accessible with semiconductor QDs and allowing for tailored radiative rates over many orders of magnitude. This extraordinary flexibility of optical properties is achieved via a combination of the spatial confinement of carriers and the strong influence of surface chemistry. The complex physics of this material, which is still being unraveled, leads to new effects, opening up new opportunities for applications. In this review we summarize the latest progress in this fascinating research field, with special attention given to surface-induced effects, such as the emergence of direct bandgap transitions, and collective effects in densely packed QDs, such as space separated quantum cutting. Keywords: silicon quantum dots, quantum dot, surface chemistry, quantum confinement (Some figures may appear in colour only in the online journal)

1. Introduction

a Si-based laser enabling full on-chip integration of electronics and photonics for optical computing [13, 14] with large data throughput at the speed of light. Furthermore, colloidal SiQDs are ideal for low-temperature deposition on any substrate (e.g., glass, plastic, wood, and textile) using techniques like printing, roll-to-roll, stamping, spraying, or painting. Given their bright color-tunable emission, colloidal SiQDs could form a basis for cheap large-area (flexible) displays, new types of lighting, and third generation photovoltaics—as ‘light paint’ or ‘solar paint’. Additionally, the nontoxicity [15–18] and biodegradability [19] of SiQDs opens up new opportunities for applications in fields with high environmental/health requirements: in-vivo bioimaging, drug delivery, lab-on-chip sensing, optoelectronic toys, smart textiles, photocatalysis for water/waste/food sterilization, phototherapeutics, cosmetics, and many more. The main disadvantages of SiQDs have up to now been the relatively poor size and/or surface definition of most of the experimentally studied SiQD systems, and a lack of sound theoretical models for anything other than hydrogen-terminated SiQDs. The effects

Silicon quantum dots (SiQDs) are in many ways very interesting materials whose full potential is still not fully appreciated, or even understood. With respect to bulk silicon, the optical and electronic properties of SiQDs are dramatically modified by and dependent on the quantum dot (QD) size. SiQD properties in general are also extremely sensitive to, and in a broad range tunable by, surface chemistry—to a level unseen in other semiconductor QDs. It has been demonstrated that SiQDs are capable of bright and spectrally tunable emission (e.g., [1–4]); in figure 1, we show examples of SiQDs emitting from the ultraviolet (UV) to the near infrared (IR) spectral region and give a comparison of the tunabilities of QDs based on various materials. Moreover, stimulated emission can be reached in SiQDs [5–12]. In this way silicon, already a dominant material in microelectronics and photovoltaics, could be considered for the development of monolithic Si photonics and optoelectronics [13]. This could lead to major shifts in the architectures of many appliances, with 0953-8984/14/173201+28$33.00


© 2014 IOP Publishing Ltd  Printed in the UK

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SiQDs [32], and enhanced emission efficiency in samples with monodisperse SiQDs [33]. We would not have to go far to find further examples. In this review, we give a summary of the latest developments in this fascinating field, with special emphasis on effects induced by surface and/or collective behavior. Also, related advances in preparation and processing techniques will be summarized. 2.  Note on terminology This review focuses mainly on SiQDs in connection with the emission of light (for a general overview of luminescence spectroscopy of semiconductors including Si nanostructures see, e.g., textbooks [22, 39]). These QDs are most commonly crystalline (‘silicon nanocrystals’), but amorphous QDs can be encountered. Nevertheless, under a certain size limit, the concept of crystallinity becomes looser due to an insufficient number of atoms and lack of long-range order (this issue can be viewed also from the point of view of the validity of the band structure concept, discussed in section 4.1). The most appropriate term for very small structures ( 1 s), probing a significant number of single SiQDs. Blinking is an additional source of emission yield losses, next to the aforementioned nonradiative trapping and phonon energy dissipation. In QD systems, a question arises whether

quenching occurred also when n-type and p-type doped powders were mixed together [322]. Also, Li doping achieved by electrochemically induced diffusion of Li to H : SiQDs and their subsequent oxidation led to a slightly blue-shifted and faster emission, interpreted as being due to Si lattice expansion and tensile strain [327]. Another interesting topic is surface doping, observed, e.g., for capping with N-type molecules [175, 328]. An in-depth study with considerable insight into the topic of surface doping is given in [124]. A separate approach is doping with lanthanides, most frequently Er for O : SiQDs in a silica matrix [329–331]. The formation of Er3+ centers in close proximity to, or on, the SiQD surface allows for an efficient energy transfer from SiQDs and consequently an efficient excitation of otherwise poorly excitable Er3+ centers. The resulting Er-related narrow-band emission is suitable for applications in telecommunications and optoelectronics [305, 332–334]. Of special importance here would be the realization of stimulated emission [335–338] at the telecommunication wavelength of 1.5 µm and high efficiency carrier multiplication for photovoltaic applications [29, 339]. 5.  Single-object and ensemble spectroscopy SiQD emission and absorption spectra, radiative rates and many other important parameters are size-dependent. Therefore, large quantities of SiQDs in colloidal samples or dense SiQD films/arrays exhibit inhomogeneously broadened spectra and stretched exponential decays. In densely packed SiQD systems, some additional effects can also emerge due to collective behavior, such as energy transfer, carrier transport and/or excitonic mobility (diffusion). To go beyond inhomogeneous spectral broadening, properties of single QDs are studied, uncovering many interesting phenomena, such as the clear occurrence of phonon replicas or blinking. 5.1.  Single QDs

Single QD spectra are typically studied with highly diluted drop casted/spin coated colloidal O : SiQDs or C : SiQDs (e.g., [34, 117, 213, 261, 340, 341]) or with sparsely positioned lithographically defined O : SiQDs (e.g., [140, 345, 346]), to ensure only a single emitter within the diffraction-limited detection area (for a comprehensive review see, e.g., [347]). The single QD spectra span a large range of phonon energies, for O : SiQDs limited to the red/NIR region [140, 213, 340, 345, 346] and covering the whole visible spectral range for C : SiQDs [34, 117, 261] (for an overview, see figure 14). Typically weak single SiQD emission is difficult to observe. In some cases, emission can be considerably quenched or enhanced by a proximal substrate or metal layer. For example, strong quenching of emission has been demonstrated by Sychugov et al [343] for SiQDs in close proximity to a Si substrate (figure 15(a)), while considerable enhancement of emission was achieved by Matsuhisa et al [77] by surface plasmon polariton mediated excitation with a proximal MgF2/Ag film. 17

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Figure 19.  Bio-imaging with SiQDs (left panel, after Shiohara et al [227], reprinted with permission from The Royal Society of Chemistry;

right panel, after Park et al [19], reprinted with permission from Nature Publishing Group).

In smaller SiQDs, energy transfer can be enhanced [360], but surface reconstruction and electronegative ligands might reduce the transfer rate significantly. Experimentally, efficient energy transfer can contribute to several effects, namely red-shifted emission, and/or carrier multiplication via SSQC. A thin oxide shell and the lower confinement of carriers in matrix embedded O : SiQDs [359] allow for energy transfer or carrier tunneling amongst SiQDs. Energy transfer is expected to occur predominantly between SiQDs of the same size or from smaller to larger SiQDs, even though this issue is still under discussion [361]. Typically, energy transfer would provide a secondary excitation channel for larger QDs, thus shifting the ensemble emission spectrum towards the red (and potentially enhancing the ‘red’ emission yield). On the other hand, excitons would aggregate in the largest SiQDs, where their population would be reduced via Auger nonradiative recombination [238], which could be further enhanced due to the fact that the largest SiQDs have a larger absorption cross-section, i.e. the formation of multiexcitons is more probable. Efficient energy transfer has also been demonstrated experimentally via efficient carrier multiplication via SSQC in closely packed O : SiQD systems in a silica matrix [29–31, 283] (figure 13(b) and (c)). In SSQC, two excitons are generated in two separated but proximal SiQDs, unlike in MEG, where the multiplied carriers occupy the same QD and are subject to fast Auger interaction. Thanks to the spatial separation, generated multiple excitons in the SSQC process have a long lifetime, determined by the emission rate. Consequently, SSQC can be studied not only via ultrafast induced absorption, detecting the number of free carriers, but also by emission QY, giving directly the number of radiatively recombined excitons from the ratio of the emitted and the absorbed photons. It has been demonstrated that QY increases in a step-like way for excitation photon energies above ∼2Eg and ∼3Eg [30] (figure 13(b)), in full agreement with carrier generation measured in the same material by ultrafast induced absorption spectroscopy [31] (figure 13(c)). SSQC could involve a phonon-assisted energy transfer process, due to the consistently observed energy loss of ∼300  meV. Theoretical simulations [283] have predicted that SSQC can successfully

the low QE of an ensemble is given by the low amount of emitting QDs or by the on average low QE of each QD (which can also be caused by the ‘off’-states [340]). For slowly emitting ‘indirect bandgap’ SiQDs it has been suggested that emission can occur only from those SiQDs with no nonradiative losses, due to the fact that the low radiative rate cannot compete with the high nonradiative rate in the system and defective SiQDs remain therefore dark [206, 238]. This could be true for O : SiQDs, where single QD measurements revealed high iQE reaching ∼90% [355]. It is however only fair to note that electroluminescence from single O : SiQDs reported a much lower iQE of ∼8–16% [356]. Last but not least, singe PL decay (single exciton lifetime) has been successfully measured, yielding (bi-)exponential dependence in C : SiQDs [117] (figure 15(d)) and O : SiQDs [349] (figure 15(c)). Especially notable also is the measured slow µs single-QD decay of O:SiQDs [237] (figure 15(c)), where the authors showed that a sum of a number of measured exponentially decreasing decays leads to stretched-exponential behavior. 5.2.  Dense arrays of QDs

In densely packed SiQD films/arrays, effects related to energy transfer or carrier transport between individual SiQDs occur. Moreover, dense packing of SiQDs might lead to stimulated emission, which is important from an application point of view and is discussed in section 6.2. Förster energy transfer (FRET) between neighboring SiQDs is inefficient for larger diameters (2–4  nm) [357], where the dipolar transitions are weak due to indirect bandgaps. In such a case, multipolar terms [358] dominate FRET at small distances and surface effects play a role in the screening. Consequently, energy transfer between SiQDs is possible only when the dots are almost in contact. In the extensive study of Luo et al [359], the importance of (i) the enhanced absorption due to band mixing, (ii) the enhanced excitonic binding energy and (iii) the miniband formation are discussed with respect to energy transfer. It was shown that the high excitonic binding energy in SiQDs is disadvantageous and that quantum confinement is not lost even for SiQDs touching each other. 18

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by several companies, such as Nanoco, QD Vision or LG and Samsung. Nevertheless, these materials are typically either highly toxic (containing elements such as Cd, As, Pb), scarce (Te, Se, In), facing questionable future availability (lanthanides) [375], or likely to pose environmental risks with longterm use (Se, Zn). On the other hand, C : SiQDs with direct bandgap transitions [4, 28] and spectrally tunable emission [4] offer an ideal environmentally friendly and readily available substitution for these materials. Apart from applications requiring current injection, SiQDs can also be used as a passive phosphor in spectral converters. These are typically combined with a blue GaN-based diode in order to achieve a ‘warmer’ color of light, similar to that of a traditional incandescent bulb, but with lower power consumption. Such a light emitter has been demonstrated with the use of a SiQD phosphor by the company LumiSands (figure 16(b)). Probably the greatest problem when designing optoelectronic components with light-emitting SiQDs is the low carrier mobility inherent to quantum-confined QD systems (see the discussion in section 5.2), which makes the traditional carrier injection very inefficient and subsequently might require special approaches. Upon direct electric injection, electroluminescence can be achieved and has been reported in various SiQD systems: porSi [45–49], silicon rich silica [376–381], SiN matrices [168, 382], SiQDs in organic materials [248, 383–385] (figure 16(a)), Er-SiQDs-SiO2 [386], and many more. Electroluminescence also exhibits size-tunability [387, 388] and has the same microscopic origin as photoluminescence [380], and therefore should be susceptible to the same engineering approach (surface chemistry, etc). Typical LED O : SiQD devices were based on MOS structures [379–381] or p–i–n juctions [389]. High external QE of 1.6% was reported from SiQDs in a SiN matrix [168] upon one directional electric injection. An alternative approach was introduced by Walters et al [390] by sequential injection of electrons and holes followed by their subsequent recombination inside the QD (bipolar injection). In these devices, the most pressing difficulty relates to the different effective barriers for tunneling of electrons and holes, leading to low external quantum efficiencies of ∼0.03% [391]. Barreto et al [392] reported that bipolar injection could also be achieved by an electrical pulse with a single polarity, with electrons and holes being injected into QDs at the beginning and end of the pulse, respectively. LEDs based on bipolar injection, fabricated using a CMOS-compatible process, can be optimized by employing multilayers with SiQDs, reaching a turn-on voltage of 1.7  V and external quantum efficiency of 0.17% [393]. The optoelectronic properties of such multilayer LEDs can be further tailored using graded sizes of QDs (thickness/composition profiling) [394]. On the other hand, C : SiQDs are used in typical organic LED schemes and have reached an external QE of 0.7% (10 V) [383] or 0.17% (5.5 V) [248] or even 8.6% (11 V) [384] in an OLED device with dodecene-terminated 5 nm SiQDs emitting at 850 nm. Such high values were attained thanks to an optimized choice of polymer transport layers (both for electrons and for holes), which ensure the confinement of electrons and holes in the

compete with MEG in smaller SiQDs closely packed in a dielectric matrix. Efficient Auger recycling and a reduced hotcarrier cooling rate observed in these types of materials [32] were shown to be greatly beneficial for the efficiency of SSQC [283]. Efficient energy transfer was also reported for SiQDs closely packed in nanospheres [362], leading to efficient upconverted emission. Close proximity of SiQDs was shown to lead to enhanced emission QY also in size-purified SiQD films [33], interpreted as an optimized electronic overlap in the wavefunctions of neighboring QDs with entropic order playing a possible role. For many applications, carrier transport is important for electric injection or carrier extraction in various devices. Carrier mobility in SiQD systems, however, is very poor, owing to high excitonic binding energies and preserved quantum confinement even for SiQDs in contact [359]. Timeresolved terahertz spectroscopy of SiQD films [363] revealed non-Drude-like (localized) behavior with high carrier backscattering. Evidence for long-range conduction between SiQDs was not found, with the free carrier lifetime being dominated by trapping at Si/SiO2 interface states. The mutual exclusion of conductivity and charge storage in dense SiQD films in silica was demonstrated [364], being very similar to the mutual exclusion of conductivity and photoluminescence, as induced by quantum confinement. The recent simulations of transport for SiQDs regularly arranged in silica [365] show that disorder seems to induce only a weak reduction of mobility, while mobility itself increases for smaller QDs by the enhanced electronic coupling. A comprehensive recent review on electronic transport in assemblies of SiQDs can be found in [366]. 6.  Prospective applications The abundance of silicon, and its non-toxicity [15, 17, 18], biodegradability [19] and compatibility with CMOS technologies [13], are some of the key advantages of silicon nanostructures. Although the material and optical properties still require some improvements and synthesis techniques are not yet well mastered, SiQDs emerge as a strong competitor to other semiconductor QD materials also for light-emitter applications [4, 28]. 6.1.  Phosphors, LEDs and displays

Si-based optical devices on a silicon microelectronics platform using CMOS-compatible technology would lead to a full integration of photonics, optoelectronics and microelectronics—new optically based microchips for optical computing. This would solve the problems with the extensive heat dissipation and the low throughput of the present-day computer chips. Moreover, the switch from microelectronics to CMOScompatible optoelectronics would not require any major changes in existing infrastructure. Many QD-based optoelectronic structures, such as LEDs or displays, have already been realized based on direct bandgap materials, see, e.g., [367–374] or the prototypes advertised 19

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emissive layer. Besides, C : SiQD OLEDs can also exhibit wavelength tunability if size-separated SiQDs are exploited, still reaching high external quantum efficiencies of ∼1% [385]. Although much higher quantum efficiencies than in O : SiQDs are achieved, the preparation techniques used to produce C : SiQD-based OLED devices are not (yet) CMOS compatible. In that case, possible solutions are C : SiQDs grown inside a silicon rich SiC matrix or N:SiQDs prepared in Si-rich SiN matrices [382]. In this context, interesting examples of QD LED structures were also samples prepared by Si/C co-implanted silica [395, 396], in which, however, SiC QDs could also have played a role.

of a photonic crystal, although this approach is still at the stage of theoretical modeling [407]. In contrast, for the blue fast (∼ns) decaying emission (F-band) in O : SiQDs, high optical gain accompanied by clear spectral narrowing and a threshold behavior was shown [12]. Unfortunately, this emission is efficiently reabsorbed by SiQDs, and, consequently, a weak slow optical gain ‘echo’ in the red spectral range can be observed [408]. To build up a laser system, the active material has to be combined with a resonator. Due to the high scattering losses in a typical SiQD system, distributed feedback cavities have been implemented [53, 409], however, with only weak spectral response. The lack of strong light amplification achieved with stimulated emission redirected interest towards other effects, such as stimulated Raman scattering used in the so-called ‘Raman laser’ [410]. A device based on this effect, however, is only a passive wavelength converter and cannot be driven electrically. Therefore, the quest for an all-silicon laser still remains an unresolved issue and the all-silicon laser might be replaced by a hybrid III–V-on-Si solution (see, e.g., [13, 411]).

6.2.  Lasers and amplifiers

The presence of stimulated emission can result in optical amplification and net optical gain, provided that the amplification outweighs possible losses. In the past, vast literature was devoted to this topic [13, 22] and positive optical gain as a result of stimulated emission has been reported from a number of O : SiQD systems in the red/orange [5, 6, 8–10] as well as the blue [7, 12, 397] emission bands. Additionally, ultrafast optical gain, attributed to the core states of O : SiQDs, has also been observed [398]. Despite numerous reports on optical gain, however, no Si-based lasers on this principle have been demonstrated up to now. Due to the generally low optical gain values, stimulated emission investigations are very challenging, as numerous ‘false gain’ artifacts can occur. Low optical gain in SiQD systems is typically investigated using the variable stripe length (VSL) technique [399, 400]. The most commonly occurring ‘false gain’ artifacts are waveguiding effects [401, 402], interference on the VSL slit [9] and generally also the non-constant light out-coupling into the detection system [403]. Most of these effects can be eliminated by the use of the shifting excitation spot technique [403], developed during the studies of gain in SiQDs and tested on porSi O : SiQDs [12, 398, 401, 402] and Er-doped SiQDs in silica [338]. Besides the experimental challenges, the main obstacles in various SiQD systems are [20] (i) free carrier losses, (ii) the lack of a four-level system, (iii) broad size-distribution or (iv) scattering of the emitted light. Considering the free carrier absorption (FCA), together with scattering on inhomogeneities, FCA is the most important source of competing losses, completely blocking lasing in bulk Si [404]. Since FCA is considerably lower for high energy photons, the loss will be lower for smaller SiQDs, with blue-shifted emission (figure 17). The second item on the list, a four-level system, advantageous for low energy threshold lasing, has been put forward for O : SiQDs, making use of the oxygen-related states [405]. The possible importance of the capping/matrix is somewhat supported by the fact that for brightly emitting SiQDs in a SiN matrix (with fast emission rate), no positive optical gain has been observed [406]. However, what is generally recognized as the main obstacle for the realization of light amplification is the broad size distribution, leading to poor energy resonance between excited SiQDs [20]. In this context, a compelling proposition is the enhancement of low optical gain values by the incorporation

6.3.  Energy conversion (solar cell, spectral shaping)

SiQDs are of great interest also for photovoltaics (PV) and energy storage. Colloidal QDs are especially interesting [414] because of the possibility of low-cost and low-temperature deposition techniques, such as spin-coating, spraying, printing, stamping, etc. The production of SiQD ‘ink’ for PV was pioneered by US start-up Innovalight in 2006 and considerable insight into the possible solutions for the production of electronic SiQD ink has been achieved only very recently by the group of Kortshagen [124]. The size-tunable optical bandgap of QDs is appealing for multijunction solar cells [415], however, the indirect bandgap does not offer as high tunable band-edge absorption as, e.g., CdSe QDs. Even in direct bandgap C : SiQDs [4], where the band-edge absorption is enhanced, the density of the modified direct states is insufficient to significantly influence the overall absorption characteristics. An additional problem arises with the efficient carrier extraction from QDs, due to the extremely low inter-dot mobility (QD/ matrix barrier) [359, 416] and high excitonic binding energy [359]. Of the various solutions generally considered for QD-solar cell architectures, only a few have been employed with SiQDs. A SiQD solar cell based on a heterojunction blend with organic semiconductors (P3HT, PCBM) has been demonstrated by Liu et al [417]. Coupling of SiQDs with carbon compounds (carbon nanotubes, graphene, fullerene) is being investigated by the group of Svrcek [85, 418], who also suggested the use of electric charge loading with microplasma treatment [85] for filling, e.g., TiO-nanotubes with SiQDs [419]. An interesting construction with alternating n and p type material structures (e.g., using n-type semiconductor QDs in a p-type polymer) was suggested by Salafsky et al [420]. For dense SiQD assembly into ‘QD solids’ for PV, SiQDs with ultrashort or no capping are required. For the latter, SiQDs with no capping were prepared via P,B codoping by the group of Fujii [324]. 20

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be tailored for the particular dye, (v) advanced bio-functionalization is possible, (vi) high two-photon absorption coefficients allow for the use of longer-wavelength excitation, with longer penetration depth and less tissue damage [440], etc. Well-developed II–VI QDs offer bright and spectrally narrow emission, but are usually toxic, which renders them unacceptable for in-vivo applications [441, 442]. An encapsulation in a protective polymer shell lowers the cytotoxicity, but leads to the accumulation of QDs in the organism. This is not the case for SiQDs, which due to low toxicity do not require any surface encapsulation and are therefore neither toxic nor accumulative. Moreover, their photostability is also superior to II–VI QDs and fluorescent dyes (figure 18, [38, 443]), shifting the interest towards them (a recent review is, e.g., [18, 444]). The very first SiQDs used as fluorescent labels were redemitting porSi-based particles [19, 445] applied to the imaging of tumors in living mice [19] (figure 19(b)). This study reported also excellent biodegradability of SiQDs into harmless orthosilicic and silicic acids (oxidation and hydration process SiO2 + 2H2O → SiH4O4) that can be excreted from the body through the urinary system. Other examples of SiQDs used for imaging include the preparation of water-dispersible red-emitting SiQDs coated with polyethyleneglycol-grafted inverse micelles and their use for imaging of live cancer cells, with no observable in-vitro toxicity [446]. Other groups then focused on red-emitting polymer-coated SiQDs [447], redemitting SiQDs terminated with carboxylic groups [38], blue/ green-emitting (in PBS) O : SiQDs and their incubation with HeLa cells [448] or SiQDs emitting in the UV/blue region [227] (figure 19(a)). Addressing the above-mentioned preference for longer-wavelength excitation/emission, in-vivo multicolor NIR imaging (tunable emission 450–900  nm) with SiQDs for tumor targeting has been demonstrated by Erogbogbo et al [449]. The use of 2- or 3-photon excitation has been studied in red-emitting styrene-grafted SiQDs [278], however, excitation power densities are too high for use in bio-systems. The low toxicity of SiQDs has been reported from various SiQD systems [15, 19, 446] and very recently, the eventual toxicity has been exclusively linked to only specific ligands [17, 18] (the choice of ligands, however, needs to fulfill the water solubility condition as well). Various types of ligands can be linked to SiQDs using the thiolene-click chemistry, which was shown to allow for complex bio-functionalization even with DNA strands [98, 99]. Some limited toxicity of O : SiQDs at high concentrations (still an order of magnitude lower than for CdSe) was reported also to arise due to the formation of oxygen radicals [448]. (However, it should be noted that simple cytotoxicity does not address all types of possible toxicity issues, and therefore neurotoxicity, genotoxicity, carcinogenicity or even teratogenicity need to be carefully assessed in the future.) The production of reactive oxygen species [450–455], though toxic for the cell, can be utilized for cancer therapy [450–453, 456, 457]. Another example of the generation of reactive oxygen species (singlet oxygen in the case of SiQDs) was a study with an outlook towards its antibacterial activity (on E. coli cells) [458]. Photocatalytic reactions of SiQDs

Besides the traditional solar cell architectures, SiQDs are interesting also for the construction of a ‘hot carrier solar cell’. As mentioned in section 4.4, in SiQDs, hot carrier relaxation is slower than in bulk Si and carrier mutliplication (CM) via MEG occurs [307, 312, 318]. However, the multiplied carriers need to be extracted very fast due to the generally short lifetime of carriers (∼ps) (for II–VI QDs see, e.g., [421], for SiQDs, e.g., [298]) if the CM effect is to be exploited. Efficient extraction has been demonstrated in other types of QDs (PbSe) using, e.g., TiO2 [422], and such QDs were employed in prototype QD solar cells [423]. This effect could theoretically also be used with SiQDs: however, in SiQDs, CM has been proven to occur also via SSQC [29–31, 283], leading to long living multiplied carriers. This effect is suitable for the spectral shaping of the solar spectrum. With SiQDs, spectral shaping can be realized via spectral shifting [424], down-conversion via SSQC [30, 339], and possibly also up-conversion. Such a spectral shaper can then be used as an additional layer in front of a standard solar cell, transforming the incident solar spectrum into a narrower, more efficiently convertable one. A general study on the enhanced efficiency of silicon solar cells with passive luminescence conversion layers is available in [425, 426]. The use of SiQDs as spectral down-convertors and shifters has been tested by the group of Pavesi and has shown a 6% relative enhancement [424, 427]. Using SiQDs in a PECVD-grown SiN thin film, it has been shown that a relative increase in the power efficiency of a crystalline silicon solar cell of up to 22.8% can be achieved [428]. Nevertheless, for applications of downconversion by SSQC in photovoltaics, the energy threshold (∼2Eg) needs to be shifted to lower energies. For this purpose, SiQDs co-doped with P and B [324] or Er-doped O : SiQDs in silica matrices [339, 429] can be utilized. The latter have been recently shown to lead to very efficient optical conversion of hot-carrier energy into the telecommunication wavelength of 1.5 µm [339, 429]. 6.4.  Data storage

Another promising application niche for QDs is in non-volatile memories. Efficient charge storage with SiQDs has been demonstrated with various MOS structures [41, 430–433]. SiQDs have also been integrated into a functional flash memory device [434] and currently, an ultrafast metal-gate SiQD nonvolatile memory has been achieved [435] with a program/ erase speed of 1 µs under a low operating voltage of 7 V. A recent review of the use of SiQDs in non-volatile memories is available here [133]. 6.5. Bio-applications

QDs in general have been proposed as an excellent alternative to organic dyes to serve as biological labels [436–439]. QDs typically outperform organic dyes in various aspects: (i) better brightness and photostability, (ii) a broad excitation spectrum, (iii) broad spectral tunability (using a single material), (iv) silanization chemistry can in principle be applied to different particles, while in organic dyes the chemistry has to 21

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were shown to be size-dependent with increasing reactiveness in smaller SiQDs [1]. Other bio-applications include bio-sensing, such as when red-emitting carboxyl-terminated SiQDs with stable photoluminescence within physiological conditions are quenched in the presence of Cu2+ (this ion can cause serious neurodegenerative diseases). The quenching has been exploited for the detection of Cu2+ in HeLa cells [459]. Alternatively, H2O2-related photoluminescence quenching can be utilized for the quantitative detection of glucose, as glucose oxidase catalysis produces H2O2 [460]. Illustrating the drug-delivery applications, an analgetic drug can be attached to the surface of SiQDs, which causes the drug's biological (pain-killing) effect to be maintained while its cytotoxicity is reduced [461]. Also of special interest are the applications of wet chemically synthesized manganese-doped SiQDs (twophoton excitation) with paramagnetic properties for magnetic resonance imaging [462]. The novelty of all these results indicates that the bio-applications of SiQDs is a dynamically evolving field, holding promise for soon-to-be real-life applications.

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7. Summary Recent progress made in the study of SiQDs is reviewed, with special emphasis on optical properties. On the twenty-fifth anniversary of the first report on light emission from a SiQD, the field is more vivid than ever, having seen essential advances and simultaneous expansion into other disciplines, and reached a broad spectrum of specialists, including engineers, material scientists, physicists, chemists and bio-physicists. We show that the progress made in preparation and processing techniques has brought about a deeper understanding of the properties of SiQDs and has also resulted in the development of materials and even devices whose functionalities are now very close to commercialization. Very specifically, this applies to the fields of bio-imaging, drug delivery, and also lighting and memory devices. Thus, silicon in the form of QDs will very soon enter areas which have until now been reserved only for the so-called ‘optically active’ materials, with clear benefits to both sustainability and the global economy. Acknowledgments The authors would like to thank Dr Jos Paulusse and Dr A N Poddubny for fruitful discussions, Prof. I Pelant for providing figure 17, Mary Levin for letting us use figure 16(b) and her prompt answer and Richard G Newell for careful proofreading. KD and TG acknowledge Stichting voor de Technologische Wetenschappen (STW), Stichting der Fundamenteel Onderzoek der Materie (FOM) and Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) for funding. KK acknowledges Czech Science Foundation grant GPP204/12/P235. References [1] Kang Z, Liu Y, Tsang C H A, Ma D D D, Fan X, Wong N-B and Lee S-T 2009 Adv. Mater. 21 661–4 22

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Silicon quantum dots: surface matters.

Silicon quantum dots (SiQDs) hold great promise for many future technologies. Silicon is already at the core of photovoltaics and microelectronics, an...
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