Fluorescent Silicon Clusters and Nanoparticles
CContents a r X i v : . [ phy s i c s . a t m - c l u s ] S e p hapter 1 Fluorescent Silicon Clustersand Nanoparticles
Klaus von Haeften
Clusters, consisting of a small number of atoms, have been in the focus ofphysical and chemical research for several decades. They often show dramaticsize effects. The addition of a single atoms can change their properties ratherabruptly because of, for example, the discreteness of shell filling (Knight et al.,1984) or sphere-packing effects (Echt et al., 1981). When clusters become largerand reach the nanometre scale, other effects are observed, such as quantumconfinement; The intense red fluorescence observed for nanostructured silicon(Canham, 1990; Cullis and Canham, 1991; Wilson et al., 1993; Lockwood, 1994;Cullis et al., 1997) is a popular and frequently cited example of this effect.The discovery of fluorescent nanoscale silicon at room temperature by Canham(Canham, 1990) increased the already quite intense research into silicon clusters2
HAPTER 1. FLUORESCENT SILICON CLUSTERS
HAPTER 1. FLUORESCENT SILICON CLUSTERS
Bulk crystalline silicon is known as a poor light emitter because of its indirectband gap. Fluorescence is a relatively inefficient relaxation process in elec-tronically excited indirect band gap semiconductors because fluorescence hasto simultaneously occur with the absorption of a phonon of matching momen-tum. This mechanism is illustrated in more detail in figure 1.1. The entireelectronic excitation and relaxation/fluorescence cycle is shown for direct andindirect band gap semiconductors.The left-hand side of figure 1.1 shows an energy band schematic of a directband gap semiconductor, characterised by the conduction and valence bandmaxima and minima being on top of each other. Photoexcitation of an electronfollows an energetic pathway indicated by the vertical arrow, reaching from thetop of the valence band an into the conduction band, from where it returns torecombine with the hole, inducing fluorescence. Photoexcitation using higherenergies is possible, but less likely because of the decreasing density of statesalong the parabola, and indeed the fluorescence wavelength will remain unal-
HAPTER 1. FLUORESCENT SILICON CLUSTERS g , the pathwaysof excitation of an electron from the valence band to the conduction band,electronic relaxation, fluorescence and absorption of phonons (see text).tered because of the relatively short timescale on which electronic relaxationoccurs in the conduction and valence bands. The fluorescence intensity will beunchanged because both electron and hole have the same momentum.This situation changes in indirect band gap semiconductors, as shown on theright hand side in figure 1.1. In indirect band gap semiconductors, the minimaof the conduction band and maxima of the valence band are shifted. As aconsequence, electrons that are photoexcited into the conduction bands quicklyundergo electronic relaxation to the minimum of the conduction band. However,any subsequent direct, vertical decay to the valence band is not possible becauseall states with similar momenta, ¯ hk , are populated with electrons; in otherwords, hole states for direct recombination with the excited electrons are notavailable. ’Diagonal’ recombination with the available, original hole state atthe maximum of the valence band is not an option because momentum wouldno longer be conserved. So, in order for diagonal recombination to happen, aphonon with matching momentum has to be absorbed during fluorescence, but HAPTER 1. FLUORESCENT SILICON CLUSTERS E ( n ), of an electron confined in a one-dimensional box oflength (cid:96) with infinitely high box walls can be straightforwardly derived to givethe following equation: E ( n ) = n π ¯ h m e (cid:96) (1.1)where n is the principal quantum number, ¯ h the reduced Planck (or Dirac)constant and m e the electron mass. The quantum number, n , is indexed from n = 1, and the energy difference E( n = 2) and E( n = 1) would be equivalent tothe fluorescence energy from the first excited state to the ground state.The analogy of the one-dimensional model can be straightforwardly extended HAPTER 1. FLUORESCENT SILICON CLUSTERS su-perlattice depended on the silicon film thickness (Lockwood et al., 1996). Theyattributed this behaviour to one-dimensional confinement of the excited elec-tron within the silicon film. To explain the shift in the fluorescence energy, theyadopted the particle-in-a-box model and showed that the peak energy of theobserved red fluorescence band followed equation 1.2. E ( n ) = E g + π ¯ h d (cid:18) m ∗ e + 1 m ∗ h (cid:19) (1.2)Here, d is the silicon layer thickness, m ∗ e and m ∗ h are the ’effective masses’ ofelectron and hole, although the authors acknowledge that, strictly speaking,the concept of effective electron and hole masses has no physical meaning innanoscale systems that do not exhibit the translational symmetry of crystals.A good fit was reported for E ( eV ) = 1 .
60 + 0 . d − eV, with d given in nm,which implies effective masses m ∗ e ≈ m ∗ h ≈ e in good agreement with bulkcrystalline silicon were m ∗ e ( Si bulk ) = 1.18 m e and m ∗ h ( Si bulk ) = 0.81 m e . Thefit shows that the band gap energy, at E g = 1.60 eV, is considerably largerthan that of bulk crystalline silicon ( E g (c-Si) = 1.12 eV at 295 K), and is, infact, more similar to that of amorphous silicon ( E g (a-Si) = 1.5 - 1.6 eV at295 K). The good fit with experimental data and the similarity between the HAPTER 1. FLUORESCENT SILICON CLUSTERS E ( eV ) = 1 .
56 + 2 . d − eV,which confirmed the earlier observed band gap energy of amorphous silicon(Lockwood et al., 1996) in the limit of large dot sizes. However, the depen-dence on quantum confinement, 2 . d − eV, was much larger. The discrepancywith work on conducted on Si/SiO superlattices was attributed to the three-dimensional confinement of the quantum dots in silicon nitride (Park et al.,2001).Summarising, we have so far seen that with reduction of size of a crystaltranslational symmetry is gradually lost with the consequence that indirect bandgap semiconductors become quasi-direct semiconductors at the nanoscale. Atthe same time the band gap energy increases because of quantum confinement.A further important factor determining the ability to fluoresce is the elec-tronic structure at the surface of nanocrystals. Surfaces break translationalsymmetry, a consequence of which is that one cannot expect the same energyband structure as might be observed for ’infinite’ crystals. Band gap energiesmay be smaller, or may not even exist. For nanocrystals, this means that surfaceeffects may compete with quantum confinement. In the following, non-radiativedecay at nanocrystal surfaces will be discussed.To prevent non-radiative decay at its surface, a nanocrystal may be embed-ded in another semiconductor or insulator of larger band gap energy. This effectis illustrated in figure 1.2.In broadest terms, one can expect that surface effects on the band gap energy HAPTER 1. FLUORESCENT SILICON CLUSTERS g and E g refer to the band gap energiesof the nanocrystal and host material, respectively.can be minimised for such a system . It is assumed that the nanocrystal structurefits well with that of the host and that a minimum of additional interface statesare produced. Ideally, this would mean that an electron promoted across E g from the valence band of the nanocrystal to its conduction band would remainconfined within the nanocrystal. The electron would have no other choice thanto fluoresce to the ground state because no discrete states are available withinthe gap.To better account for non-radiative decay in real systems, vibrational relax-ation is often considered. Because of the possibility of surface reconstruction,vibrational relaxation at surfaces is particularly important for free nanoscalecrystals and clusters. Figure 1.3 illustrates the cycle of excitation, electronicmigration and relaxation within the conduction band and vibrational relax-ation at the surface. In this simple picture, a high density of vibrational states HAPTER 1. FLUORESCENT SILICON CLUSTERS
HAPTER 1. FLUORESCENT SILICON CLUSTERS inf h , C v and D h , can be identified, with the actual point group de-pending on the amplitude of the vibration. In other words, by performing abend vibration, the molecule is able to intersect the different electronic statesassociated with these point groups. These intersection points provide passagesto lower-lying states.These lower states may be dissociative. Therefore, a tendency towards iso-merisation and dissociation may be expected, particularly for small homonuclearclusters. This corroborates the relevance of caging through a rigid shell of atoms,moieties or solvent molecules. In fact, deposition of metal clusters into argonmatrices and argon droplets has made the observation of photoluminescencepossible (Felix et al., 2001; Sieber et al., 2004; Conus et al., 2006; Harbich et al.,2007). Argon shells have also been used to cage oxygen molecules; the observedluminescence was attributed to oxygen atoms that had first dissociated but werethen forced, by the cage, to recombine (Laarmann et al., 2008).In summary, non-adiabatic decay at the surface of homonuclear clusters isan important relaxation channel in terms of its competition with fluorescence.The design and engineering of fluorescent clusters therefore requires that this HAPTER 1. FLUORESCENT SILICON CLUSTERS
Clusters are understood as being particles consisting of as few as two, three, orfour, or as having as many as a few thousand, atoms. The term ’cluster’ is usedalongside the term ’nanoparticles’, but clusters are commonly understood torepresent smaller particles. The study of clusters is motivated by the desire tounderstand the often dramatic changes of material properties that accompanychanges in size. Material properties also depend on structure and dimensionality,and similarly accompany changes in size. All such characteristics are relevantin cluster science research.
HAPTER 1. FLUORESCENT SILICON CLUSTERS Free clusters are frequently studied in supersonic beams. A gas under highpressure is expanded through a tiny aperture into vacuum. During expansion, itcools and nucleates via removal of excess energy through three-body collisions,forming clusters. They fly through vacuum where they can, for instance, beinvestigated free from external interactions, or can otherwise be deposited ona substrate. For silicon clusters, this production method is unsuitable becauseit requires atomic vapour of significant pressure. Silicon has a comparably lowvapour pressure, even at high temperatures, rendering supersonic expansion ofthermally produced silicon vapour through a nozzle into vacuum an unrealisticmechanism for silicon cluster production.Supersonic beams have been employed in sources where silicon has beengenerated by decomposition of precursor gases which are then mixed with acarrier/aggregation gas. Silane diluted in helium and exposed to a dischargehas been been expanded through a pulsed piezoelectric valve (Hoops et al.,2001). Ehbrecht et al. used a pulsed CO laser to decompose silane dilutedin helium (Ehbrecht and Huisken, 1999). The products were expanded intovacuum, and subsequently investigated by time-of-flight mass spectrometry ordeposited onto CF or LiF substrates (Ehbrecht et al., 1997).Silicon clusters have been produced using the principle of gas aggregation(Sattler et al., 1980). Silicon vapour is mixed with a ’seeding’ gas, which inducesthe three body collisions required for nucleation. It also facilitates growth bymediating collisions between silicon atoms and removing the latent heat that isreleased during any subsequent growth of clusters from these collisions.To generate silicon vapour within a seed gas, laser vaporisation, sputteringand pulsed arcs have been used. Laser vaporisation sources employ pulsed lasersources that are fired at a rotating silicon rod (Bloomfield et al., 1985; Jarrold HAPTER 1. FLUORESCENT SILICON CLUSTERS on highly orientedpyrolytic graphite (HOPG) and performed X-ray photoelectron spectroscopy(Grass et al., 2002). Astruc-Hoffmann and co-workers used a reflectron time-of-flight (RETOF) mass spectrometer in combination with a multi-wire mass gateto size select silicon anion clusters on which to subsequently perform photo-electron spectroscopy. The size-selected clusters were irradiated with UV lightfrom an ArF excimer laser and analysed in a magnetic bottle photoelectronspectrometer (Astruc Hoffmann et al., 2001a). HAPTER 1. FLUORESCENT SILICON CLUSTERS
A great variety of geometric structures have been reported for silicon clustersusing both theoretical and experimental methods. in general, it has been foundthat the structures of neutral, cation and anion cluster species differ consider-ably. Anions have frequently been used to elucidate electronic and geometricfeatures. Their structures are affected by Jahn-Teller distortions. Care must betaken when compared to neutral clusters.In a number of early studies, silicon clusters produced using supersonicbeam techniques were deposited and investigated spectroscopically. Honea andcoworkers deposited size-selected silicon clusters into argon, krypton and nitro-gen matrices using co-deposition onto a liquid helium-cooled substrate (Honeaet al., 1993b, 1999). Using surface-plasmon-polariton-enhanced Raman spec-troscopy, sharp features characteristic of Si , Si and Si structures were iden-tified in their spectra, which were in good agreement with earlier ab initio cal-culations (Raghavachari and Logovinsky, 1985; Raghavachari, 1986; Tom´anekand Schl¨uter, 1986; Raghavachari and Rohlfing, 1988; Ballone et al., 1988). Thespectra also revealed evidence for the presence of cluster-cluster aggregationwithin the rare gas matrix. The spectra of the aggregates of Si , Si and Si bore considerable similarities to those of larger clusters, such as Si − , as wellas those of amorphous silicon (Honea et al., 1999).The structure of free silicon clusters in the size range from n = 10 - 100 wasaddressed using drift mobility measurements, the results of which revealed a HAPTER 1. FLUORESCENT SILICON CLUSTERS − where siliconclusters absorb. Vibrational spectroscopy of small silicon cluster cations wasperformed using multiple photon dissociation spectroscopy. The ions were anal-ysed in a time-of-flight mass spectrometer. Also, isotopically selected Xeatoms were attached to the clusters. Absorption of multiple IR photons wouldlead to a depletion of the ion signal, allowing the requisition of spectra of size-selected clusters. Comparison of the spectra with calculations using densityfunctional theory revealed novel structures and a growth motif that startedwith a pentagonal bipyramid building block and changed to a trigonal prism forlarger clusters (Lyon et al., 2009).Related work provided information on the structure of small neutral sili-
HAPTER 1. FLUORESCENT SILICON CLUSTERS − . The use ofvacuum-ultraviolet two-color ionisation provided the advantage of detection ofthe initially neutral clusters with mass selectivity. An increase of the ionisa-tion rate was observed when IR photons had been absorbed. Comparison withdensity functional theory (DFT) and Møller-Plesset (MP2) perturbation the-ory calculations revealed that the ground state potential energy surface wasvery flat. Therefore, rapid interconversion between different structures mightwell be expected, as well as the presence of higher-energy isomers in real-worldexperimental samples (Fielicke et al., 2009).Furthermore, theoretical work suggested that, at intermediate sizes, around20 atoms silicon clusters tend to build irregular cages stabilised by a small num-ber of encapsulated silicon atoms (Mitas et al., 2000). Silicon cages can also bestabilised by encapsulated foreign atoms (Kumar and Kawazoe, 2001, 2003b,a).Stable hollow structures, similar to C buckminsterfullerene (buckyballs), cannevertheless be excluded (Sun et al., 2003).In summary, the structures of silicon clusters grown in the gas phase differfrom the structures of silicon nanocrystals that have been produced, for ex-ample, by etching of bulk crystalline silicon. The structures of small siliconclusters are characterised by the tendency of the atoms to minimise coordina-tion, thereby favouring growth of prolate shapes. Therefore, their bond anglesare smaller than their counterparts in sp bonded, cubic diamond-structuredbulk silicon. The consequences of this behaviour are shorter internuclear sep-arations and higher atomic densities. An exemption from these principles isshown in the work of Laguna and co-workers, however, who deposited silicon HAPTER 1. FLUORESCENT SILICON CLUSTERS
Photoelectron spectra of silicon cluster anions have frequently been reportedin the literature. These spectra exhibit rich features for small clusters, whichbecome smoother as the clusters become larger. Maus, Gantef¨or and Eberhardt(Maus et al., 2000) assign the low binding energy features to the extra elec-tron occupying the conduction bands. The higher energy features observed areattributed to the valence electrons. The energy difference between the two corre-sponds to the band gap in the bulk picture. Small clusters between n =3 and 13were found to have band gap energies smaller than those typically seen for bulkcrystals (Maus et al., 2000). This is incommensurate with the idea of quantumconfinement, which would predict larger band gap energies for anything smallerthan the bulk. The results were attributed to the entire geometric and electronicstructure being affected by surface effects, similar to the reconstruction of thesurface of bulk silicon crystals. While such an effect must clearly dominate theelectronic structure of small clusters, the trend continues for larger clusters aswell; Hoffmann et al., for instance, report the absence of a band gap for clustersup to 1000 atoms (Astruc Hoffmann et al., 2001b). For large Si cluster anions,the photoelectron spectrum is dominated by a single smooth and broad feature.The onset of photoemission shifts with size towards larger binding energies, atrend that is incommensurate with a bulk band gap picture and contrary towhat one would expect from quantum confinement.The observations made through photoelectron spectroscopy of cluster anions HAPTER 1. FLUORESCENT SILICON CLUSTERS + n were determined by X-ray spectroscopyusing monochromatic synchrotron radiation and an ion trap to store the size-selected clusters (Lau et al., 2011). By measuring the ion yield of specific iondecay channels, it was possible to record a direct 2p photoionisation spectrumseparately from the resonant 2p photoionisation spectrum, yielding the energydifference E XAS between the core level and the lowest unoccupied molecular or-bital (LUMO,) and E CL , the energy difference between the core and the vacuumlevel, E V AC . Measurement of the valence state photoionisation spectrum E V B yields the difference between the highest occupied molecular orbital (HOMO)and the the vacuum level, E V AC . For the band gap energy, E g , or, more pre-cisely, the HOMO-LUMO energy difference, it follows E g = E V B − E CL + E XAS .Photoionisation thresholds were measured for silicon clusters in the sizerange from n = 2 to 200 using laser photoionisation using RETOF mass spec-trometry detection (Fuke et al., 1993). The ionisation potential revealed fea-tures that were ascribed to a structural transition for sizes around n = 20.Measurements of the 2p core-level and valence electron binding energies usingmonochromatic synchrotron radiation and an ion trap show a similar size de-pendence (Vogel et al., 2012). Both 2p binding energy and ionisation potentialshow a linear dependence on the inverse cluster radius n − / . Such a depen-dence is expected from the size-dependent charging energy, similar to metalclusters (Halder and Kresin, 2015). Furthermore, core level shifts were derived HAPTER 1. FLUORESCENT SILICON CLUSTERS
The electronic level structure associated with dense packing, suggesting verysmall band gap energies for small- and medium-sized silicon clusters, and eventhe absence of band gaps, is unfavourable towards fluorescence. Indeed, fluores-cence from free silicon clusters in traps or in molecular beams has not yet beenreported in the literature. The work on neutral and cationic silver clusters inargon droplets and solid matrices (Felix et al., 2001; Sieber et al., 2004; Conuset al., 2006; Harbich et al., 2007) suggests that fluorescence might be possi-ble if silicon clusters are deposited and embedded in rare gas matrices. Whilesuch deposition experiments have been carried out (Honea et al., 1993b, 1999),attempts to observe fluorescence with this setup are not known to the author.It appears that fluorescence reported for nanoscale silicon can be attributedto the effects of quantum confinement, passivation, and the presence of defects.To achieve confinement, passivation or defects due to other materials, moleculesor atoms are actively introduced. In the vast majority of bottom-up methodsused to produce fluorescent silicon nanoparticles, chemical methods are em-ployed. Exceptions are laser vaporisation of silicon targets in liquids (ˇSvrˇceket al., 2009a,b; Intartaglia et al., 2012a; Alkis et al., 2012; ˇSvrˇcek et al., 2016;Rodio et al., 2016), pyrolysis of silane in gas-flow reactors (Ehbrecht et al., 1997)and silicon cluster co-deposition with water vapour (von Haeften et al., 2009).
Silicon clusters produced by CO laser-induced decomposition of silane werefound to show red photoluminescence after they were deposited onto LiF or HAPTER 1. FLUORESCENT SILICON CLUSTERS substrates and transferred to ambient air (Ehbrecht et al., 1997). Owingto a continuous supersonic beam and a pulsed CO laser, the part of the beamcontaining clusters was also pulsed. A velocity selector was employed to selectvelocity segments of the cluster pulse, and the cluster mass was measured bytime-of-flight mass spectrometry (Ledoux et al., 2002). The average number ofatoms in the clusters, N , was found to vary from N = 395, corresponding to anaverage diameter, D , of 2.47 nm to N = 9070, corresponding to D = 7.03 nm.The diameters were deduced using a spherical particle model, D ( N ) = (cid:18) N π V unit (cid:19) / (1.3)where V unit = 0.1601 nm is the volume of the unit cell of bulk crystalline silicon.Equation 1.3 takes into account the fact that the unit cell has a diamond cubicstructure and contains eight atoms. It is assumed that bulk and nanoparticledensities are the same.These samples were photoexcited at 488 nm using continuous laser radiation,and the resultant fluorescence spectrum was measured. Each sample showedan almost symmetric fluorescent band whose peak wavelength decreased withparticle diameter. The size-dependent fluorescence wavelength shifts agreedwith the results one might anticipate from quantum confinement. Deviationswere attributed to partial oxidation of the surface layer, which could also be seenin high resolution transmission electron microscopy (HRTEM) images (Lagunaet al., 1999). The oxide shell thickness could be correlated linearly to the clusterdiameter. The smallest clusters of 6 nm in diameter had an oxide shell witha thickness of 0.81 nm, whilst the largest particles had a diameter of 34 nmhad a 3 nm thick oxide shell (Hofmeister et al., 1999). The HRTEM imagesshowed that the nanoparticles had a crystalline core. A number of differentsilicon lattice planes were identified from diffraction rings. HAPTER 1. FLUORESCENT SILICON CLUSTERS d (111) = 0 . D + 0 . nm ] (1.4)where D is the cluster diameter.Because the photoluminescence of silicon was found to depend on pressure,the authors concluded that the size-dependent lattice separation must be takeninto account in a modified equation for the photoluminescence energy causedby quantum confinement (Ledoux et al., 2000). E corrP L = E + 3 . D . + 0 . D − .
245 (1.5)Here, E corrP L is the energy of the photoluminescent band, as corrected forsize-dependent lattice separation, in eV. E is the band gap energy of bulkcrystalline silicon at room temperature (1.17 eV) (Ledoux et al., 2000).It is important to note that all samples had been transferred to air testingfor photoluminescence. The samples were kept in an argon atmosphere duringtransfer (Ehbrecht et al., 1997). After production and exposure to air, the crys-talline core section of the particles was found to reduce in diameter (Ledouxet al., 2000, 2001). Also, the photoluminescence energy was found to blue-shift,which was attributed to the smaller sizes of the silicon clusters, supportingthe assertion that quantum confinement was controlling fluorescence properties(Ledoux et al., 2001). The effect of quantum confinement was also investigated HAPTER 1. FLUORESCENT SILICON CLUSTERS ). This was found to reducethe cluster size and the intensity of visible luminescence. To investigate the effect of passivation of silicon clusters in situ, von Haeftenand co-workers used a molecular beam co-deposition scheme (von Haeften et al.,2009). They produced silicon clusters by gas aggregation using ion sputtering inan argon-helium atmosphere, co-depositing them with a beam of water vapouronto a liquid nitrogen-cooled target. After a deposition time of 30 minutes,the target was warmed up, whereupon the ice-silicon mixture melted and a fewmillilitres of liquid sample was collected. A schematic of the apparatus used isshown in figure 1.5.When photoexcited with 308 nm UV light, all liquid samples showed a bluefluorescence that peaked at 420 nm (von Haeften et al., 2009). The fluores-cence intensity was stable over several months (Brewer and von Haeften, 2009).When the photoexcitation wavelength was decreased from 310 to 240 nm, thewavelength of the fluorescent band remained at 420 nm; however, additionalfluorescence bands appeared in the UV region (von Haeften et al., 2010a,b; Tor-ricelli et al., 2011). When the clusters were embedded in liquid ethanol and
HAPTER 1. FLUORESCENT SILICON CLUSTERS → S tran-sition observed for two-fold coordinated Si in SiO (O – Si – O) (Skuja et al.,1984; Skuja, 1992; Nishikawa et al., 1992; Fitting et al., 2004).Short lifetimes have frequently been reported for the blue fluorescence ofnanoscale silicon (Kovalev et al., 1994). Tsybeskov and co-workers investigatedthe lifetime of blue fluorescence emitted from thermally and chemically oxidisedporous silicon (Tsybeskov et al., 1994). The decay was multi-exponential with a HAPTER 1. FLUORESCENT SILICON CLUSTERS ∼ HAPTER 1. FLUORESCENT SILICON CLUSTERS Cand increased in intensity up to temperatures of 1050 C, after which no fur-ther increases were seen (Kontkiewicz et al., 1994). Fourier transform infrared(FTIR) measurements showed the presence of silicon oxide. This preparationwas also repeated in a nitrogen atmosphere. Annealing in nitrogen did not pro-duce blue luminescence. This led to the conclusion that the blue luminescenceoriginates from oxidised nanostrutured silicon, although later work showed a cor-relation between blue fluorescence intensity and nitrogen content (Dasog et al.,2013). Results similar to those of Konkievicz and co-workers have also beenreported from porous Si that was oxidized and annealed at 880 C (Kanemitsuet al., 1994). Depending on the production method, the band maxima rangefrom 400 to 460 nm (Yu et al., 1998). The specific response of red and blueluminescence intensity to repeated etching and oxidation was investigated byLockwood and co-workers, leading them to the conclusion that, at least, quan-tum confinement cannot be responsible for the blue luminescence (Lockwoodet al., 1996).An important aspect of chemical treatment is how silicon nanoparticles in-teract with an aqueous environment. This is because water is a strong oxidisingagent, but also because of the relevance of nanoparticles to biomedical applica-tions. Water may also be expected to quench fluorescence because of its dense
HAPTER 1. FLUORESCENT SILICON CLUSTERS − )and silicon dihydride (2120 cm − ) was correlated with the appearance of bluefluorescence. Prior to treatment with water, both lines were present in thespectrum, and decreased in intensity after boiling water was added. Instead aband appeared at 1105 cm − , showing that treatment with water had causedoxidation (Hou et al., 1993).Koyama and co-workers annealed oxidised porous silicon in water vapourand observed a drastic enhancement in the blue fluorescence intensity (Koyamaet al., 1998). Infrared absorption spectroscopy indicated that this annealingincreased the absorption peaks related to OH vibrations except for those of freesilanol, which disappeared completely. No traces of carbon-related signals wereobserved, contrary to the previously suspected involvement of carbon (Kon-tkiewicz et al., 1994; Canham et al., 1996).Many authors report that the emission of blue fluorescence is correlatedwith very small structures, perhaps only a few nanometres in size. Akcakirand co-workers etched p-type boron-doped silicon using H O and HF (Yamaniet al., 1997; Akcakir et al., 2000). The combined effect of the two chemicals HAPTER 1. FLUORESCENT SILICON CLUSTERS − (SiH scissors or SiH ),880-900 cm − (Si-H wagging) and 2070-2090 cm − (SiH stretch and coupledH-Si-Si-H). The 1070 cm − Si-O stretch was also observed. Treatment withH O and subsequent IR spectroscopy was found to replace first the di- andtrihydrogen bonds, and then the Si-H with Si-O. The coupled H-Si-Si-H bondsshowed somewhat greater resilience. The blue fluorescence intensity changed byno more than a factor of two after H O treatment (Belomoin et al., 2000).Fluorescent silicon clusters produced by co-deposition with water vapouronto a cold target showed a similar sizes. Atomic force microscopy in non-contact and constant force mode of cluster films produced by drop-casting col-loidal solution onto freshly cleaved HOPG showed uncovered regions of graphite,and agglomerated monolayers, as well as double layers, of clusters (Torricelliet al., 2011). The height of the monolayers reflected the difference of the tip-HOPG and tip-cluster forces, and hence cannot be taken as a measure of clusterheight. However, measuring the differences between the tip-first cluster and tip-second cluster layers was expected to give a fair estimate of the height of theclusters in the film. Values between 0.92 and 1.62 nm were found (Galinis et al.,2012a), in very good agreement with Belomoin and co-workers (Belomoin et al.,2000). HAPTER 1. FLUORESCENT SILICON CLUSTERS
Silicon clusters consisting of a small number of atoms are fascinating objectsthrough which one can study the evolution of material properties with complex-ity and size. Free clusters produced in molecular beams have properties thatare unfavorable for light emission. However, when passivated or embedded ina suitable host, they may emit fluorescence. The current available data showthat both quantum confinement and localised transitions, often at the surface,are responsible for fluorescence. By building silicon clusters atom by atom, andby embedding them in shells atom by atom, new insights into the microscopicorigins of fluorescence from nanoscale silicon can be expected.The methods needed to perform such experiments, such as spectroscopy indroplets of argon (Felix et al., 2001) and helium (Feng et al., 2015; Katzy et al.,2016), have recently been developed. It can be hoped that they will be used
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