| Silicon is the fundamental material of modern microelectronic devices and solar cells.Opened with the birth of first transistor in 1947,followed by further refinement of integration circuit in 1958 and later CMOS device,semiconductor technology has been rapidly evolving in past 70 years.During this period,humans enjoyed the far-reaching improvements in their life style resulted in the progress of information tools like computers and cell phones,which are all achievements of mature silicon-based planar process.In past 20 years,the development of integration circuit well fulfilled the prediction of Moore's Law,as the chip size keeps decreasing and the number of transistor on one chip keeps increasing.As the newest progress,Intel released the new generation CPU with a gate length of 14nm in 2013.Unfortunately,this chip size has been close to the "physical limit",as further decreases will lead to severe problems like tunneling effects,RC delay or overheating.One of the strategies to overcome this problem is using photons as the alternate of electrons as signal-carriers in chip,for their much quicker response,wider bandwidth,and absence of RC delay.That means the integration of silicon-based light source,waveguide,amplifier,modulator and detector on one single chip.By now,researchers have achieved most integration of these devices,except the light source.That is mainly due to the low emission efficiency of crystalline silicon with an indirect band-gap.In another aspect,95%commercial solar cells are made of silicon today.A theoretical 31%limit to the power conversion efficiency of a single-junction silicon solar cell is well-known as "Shockley-Queisser limit",which is mainly induced by the mismatch between solar spectrum and silicon absorption.As a result,the obtained efficiency of solar cells today is far below the necessary value for beating conventional fuel energy if consider their costs.To sum up,both the demands of high-performance information tools and high-efficiency green energy resource push us to investigate new methods to improve the optoelectronic properties of silicon material.In this manuscript,we aim to synthesis silicon-based optoelectronic materials with novel absorption and/or luminescence properties by selecting several strategies,including the fabrication of silicon nanostructures and introduction of doping process.For instance,colloidal nanocrystals(NCs)are known have different physical,chemical and optoelectronic properties compared to their bulk counterpart.The promise of these materials arises from both the potential benefits of quantum size effects and the practical advantages of solution-phase processing.Quantum size effects enable tuning of both the absorption onset and emission wavelength of NCs over a wide range,while solution-phase printing and coating processes enable large device area,use of low-cost and flexible substrates,and reduced costs in device fabrication compared to conventional vacuum processing.Another strategy is using rare-earth ions like europium and erbium as fluorescent dopant to silicon or silica material for their sharp and stable luminescence properties due to their special 4f-4f transition.Furthermore,the obtained materials are used to fabricate prototypical optoelectronic devices by some convenient solution phase process like spin coating.The optical or optoelectronic properties of these prototypical devices are measured in detailed on both the purposes of examining their practical performances and evaluating the potentials of the used silicon-based materials in practical applications of new generation optoelectronic devices like silicon-based light source on chip and high-performance photovoltaic devices.The main content of the manuscript is listed as follows:1.Despite the fact that bulk silicon is a very poor light-emitter,silicon-based nanostructures,like porous silicon and silicon rich silicon oxides(SROs),exhibit size-dependent optical and electronic properties,and unusual high-efficient luminescence.Colloidal II-IV and VI-IV group semiconductor NCs,like CdSe and PbS,have achieved tremendous success in device fabrication these year for their compatibilities with solution process.Compared to these NCs,silicon NCs offer potentially important advantages in terms of cost and,especially,low toxicity.However,they are much less amenable to solution phase synthesis than CdSe and other compound semiconductor NCs.Thus,developing improved methods of preparing these NCs and modifying their surfaces is an ongoing research challenge.A three-step process,combining laser-induced gas-phase crystallization of silicon NCs,solution-phase etching and UV-induced catalytical reactions,was used to fabricate high-quality functionalized and size controllable colloidal silicon NCs.First of all,a laser-pyrolysis reactor was used to synthesis spherical well-crystalline silicon NCs with a mean size of 15nm from splitting silane gas.After parameter optimization,the production rate of silicon NCs was up to 0.5gram/hrs;Secondary,smaller silicon NCs with controllable sizes from 15-1.5nm were obtained by solution phase etching using hydrofluoric acid and nitric acid mixture.These silicon NCs illustrate clean,oxide-free Si-H bonds on their surface and strong size-dependent emission ranging from 700-400nm.By detailed analysis of the relationship between particle mean sizes and emission wavelength,these emission procedures were ascribed to the band-to-band recombination of carriers in silicon NCs.Followed by this,the bare silicon NCs were functionalized with appropriate ligands by using UV-induced hydrosilylation reaction.By introducing different ligand precursor into the reaction,fabrication of silicon NCs with different hydrocarbon chain length on surface,different solubility in organic solvents and different physical and chemical properties can be realized.On the same time,Surface functionalization by long organic ligands like ethyl undecylenate strongly improved the colloidal stability of silicon NCs in certain solvent and the stability of luminescence derived from silicon NCs in air.2.Silicon NCs are typically capped with long organic ligands that prevent aggregation of particles,stabilize them against oxidation,and allow formation of stable high-concentration dispersions(inks)in organic solvents.These properties allow the NCs to be used in solution-based device fabrication.Unfortunately,ligands with long hydrocarbon chains,such as octadecene or ethyl undecylenate,may block interparticle charge carrier transport,which results in exceedingly low charge mobility in solid films of the particles.However,direct synthesis of NCs passivated by short ligands is often ineffective in preventing NC aggregation,resulting in formation of rough,cracked,porous,or otherwise defective NC films fabricated from colloidal NC solution.The solid-state ligand-exchange strategies that are commonly applied to improve charge mobility in ?-? and ?-? NC thin films cannot generally be used with silicon NC films,because the ligands on silicon NCs are typically attached by strong covalent bonds.These strong covalent bonds are desirable for providing robust protection from oxidation,but have the disadvantage of precluding ligand exchange in cast films.Thus,on the purpose of achieving silicon nano-ink with high colloidal stability without overly diminishing the conductivity of Si NC solid films cast from those dispersions,an appropriate surface ligand has to be carefully selected.In this work,we used diallyl disulfide(DADS,CH2=CH-CH2-S-S-CH2-CH=CH2)as the precursor of surface ligand and the solvent of silicon nano-ink.Compared to the ligands usually used in stabilizing silicon NCs,diallyl disulfide has a relatively short hydrocarbon chain,and two terminated alkene group providing the possibility of passivating the Si NC surface by hydrosilylation.The lone pairs of electrons in disulfide(S-S)bonds may favor the charge transport among the Si NCs.Moreover,this molecule has relatively high viscosity compared to the conventional organic solvents used for solution-processing in optoelectronics,e.g.,hexane,toluene and dichlorobenzene.As a result,casting films from DADS as solvent improves film smoothness and reduces crack formation.We found that as-etched Si NCs(?30 mg)with Si-H surface could be directly dispersed in DADS(1 ml)to form colloidal silicon nano-ink.Such colloidal dispersion was not achieved in the previous investigation of surface Si-H terminated Si NCs using ethyl undecylenate as solvent.UV-induced hydrosilylation was then used to stabilize the NCs against oxidation in air.The above functionalized silicon nano-ink with the concentration around 30mg/mL was used directly to fabricate silicon-NC-based optoelectronic device by a solution-processing spin coating method in air.The designed structure of device firstly consisted of simply ITO-Si NC-aluminum stacks.A Schottky junction barrier was expected to form at the interface between the Si NC layer and A1 back electrode.After that a PEDOT:PSS layer with a thickness of 25-30nm was added between ITO layer and Si NC layer to improve the carrier separation on the front light surface.The current-voltage(J-V)curves for the device were measured in the dark and under illumination of simulated solar light.The open circuit voltage of?0.25 V demonstrates the photovoltaic behavior and confirms formation of a Schottky barrier at the junction between Si and Al.To test the transient photoresponse,the current density in the device was measured as the light was switched on and off.All of the measurements were carried out in air without any device packaging or protection of inert gas.The results turned out that the photodiode exhibit very quick response to the light.Introduction of PEDOT:PSS layer strongly improved the transient photoresponse of the photodiode.The optimal working bias voltage decreased from 10V to 0.1 V,and the ratio of photocurrent under solar illumination to the dark current increased from 3.5:1 to 700:1.The photodiode also demonstrated much better air stability,no decay of the photocurrent was observed even after tens of on/off circles.The full spectrum photoresponsivity of the device was measured by monitoring the current as the wavelength of incident light was varied.No photoresponse was observed for wavelengths longer than 610 nm,consistent with a quantum-confinement induced blue-shift of the indirect band gap energy.The device illustrated narrow and strong photoresponsivity at UV region,with a maximum photoresponsivity of 0.02A/W at 310nm.This strong response was attributed to the first direct transition of Si NCs.To test the stability of our devices,we stored them at ambient conditions and measured the spectral responsivity after one month.The peak responsivity was?0.011 A/W,within a factor of 2 of the initial responsivity,and it demonstrated no further decline.This demonstrates exceptionally promising air stability for an unpackaged device created from NCs with high surface area.Nevertheless,such a "visible-blind' photoresponse makes the photodiode valuable for a number of applications in both civilian and military arenas.It also provides the potential of fabricating high-performance deep UV photodiode based on colloidal Si NC dispersion by utilizing the quantum confinement of crystalline silicon.Furthermore,the high performance of prototypical photodiode also indicates that the applied silicon nano-ink functionalized by DADS has the potentials in fabricating a tandem structure with conventional crystalline silicon solar cell to reduce the relaxation loss of incident UV photons.3.The field of luminescence from rare-earth ions has been one of steady growth during the past decade,principally due to the ever-increasing demand for sources that can be readily integrated with existing fiber and silicon microelectronics technology.In this context rare-earth doped silica have been areas of particular importance.The fortunate coincidence between the Er3+ emission band around 1540nm and the principal low-loss window in the absorption spectrum of silicate optical fiber has been the main driving force behind much recent work on optical fibers and waveguides.There is recently a significant interest in planar waveguides for use in optical integrated circuits.Such integration is driven by the increasing interest in applications as Wavelength Division Multiplexing(WDM),optical computing and planar lasers.Unfortunately,the small excitation cross-section of rare-earth ions in a Si02 matrix(typically around 10-21cm2)leads to insufficient emission efficiencies of these materials.A proven effective strategy to overcome this problem is co-doping with another material with higher excitation cross-section to sensitize rare-earth ions.SRO is a typical example.This material consists of silica doped with excess silicon in the form of nanometer-sized clusters or crystallites,which can act as sensitizers to promote rare-earth emissions by harvesting the excitation photon energy and then transferring it to the rare-earth ions.However,there is a strong thermal quenching of luminescence in this material due to the back energy transfer from rare-earth ions to silicon NCs with small band-gaps.Therefore,metal oxide NCs with wider band-gaps like ZnO,In2O3 and SnO2 can be used instead of silicon NCs in order to avoid the back energy transfer effect.Furthermore,these environmentally friendly materials have very stable chemical stabilities and are compatible with solution phase synthesis in aqueous solution,which enable the synthesis of NCs and rare-earth co-doped silica thin films by convenient solution phase chemical methods like sol-gel process.In this work,freestanding SnO2 NCs with uniform size and well crystallinity were firstly prepared by using a solution phase chemical technique at ambient environment.Surfactant was used to control the growth and agglomeration of the SnO2 NCs.As-synthesis SnO2 NCs demonstrated mean particle sizes around 2.8-3.6nm that were slightly dependent on the reaction time and temperature.Post-annealing with different temperature in the range of 400-1000 ? further enlarged particle sizes ranging from 3-20nm.This method is practically compatible with synthesis of other metal oxide NCs like ZnO or In2O3,as their corresponding hydrates share similar amphoteric properties.The optical band-gap of SnO2 particles was enlarged compared to its bulk counterpart and the red-shift of the optical band-gap with the particle size was observed which can be attributed to quantum confinement effect.A broad photoluminescence band in the range of 350-550 nm associating with the defect states on the SnO2 NC surface was detected,while a size-dependent excitation maximum similar with optical band-gap was also observed.At the same time,a sol-gel approach and subsequent thermal treatment were used to fabricate metal oxides(ZnO,In2O3 or SnO2)NCs doped silica thin films.The metal oxide NCs were uniformly formed in situ during post-annealing treatment.We found that post-annealing treatment at the range of 900 to 1000? is an effective way to reduce the hydroxyl content in silica matrix and improve crystallization of metal oxide NCs.Based on these,the number density and mean sizes of NCs were well controlled by varying the metal salt amounts in the sol precursor.The particle sizes of NCs obtained with optimal parameters were close to the Bohr radii of these materials,so a size-dependent band-to-band absorption of UV photons ascribable to quantum confinement was observed.The excited electrons were trapped by defect states on NC surface,inducing broad-band visible emission related to surface defects.These spectral properties were similar to the one of freestanding NCs.4.Rare-earth ions(Eu3+ or Er3+)were co-doped with metal oxide NCs In2O3 or SnO2)in silica using sol-gel process and post-annealing treatments.Other than metal salt that formed embedded nano-sized crystallites,rare-earth was found uniformly dispense in silica matrix as ions without aggregating into clusters even at a post-annealing temperature up to 1000?.The highest concentrations of activated rare-ions in gel-silica were measured as 2-3mol%,which preceded most rare-earth doped silica materials synthesized by ion-implantation or sputtering.The characteristic emission intensities of both used rare-earth ions were found enhanced by two orders of magnitude.The defect-related emissions of metal oxide were quenched at the same time.The optimal excitation wavelength was not restricted at some sharp peaks ascribable to rare-earth characteristic transitions anymore and substituted by broader and stronger bands located at UV region.These results indicated that the excitation approach of rare-earth changed.These excitation bands blue-shifted with decreasing NC sizes,which well accorded with the variety of excitation band related to defect-state emission in only-NC doped silica samples.So we ascribed them to the band-to-band transition in NCs whose band-gaps are enlarged by quantum confinement.Quantitative studies to the NC concentration dependences of rare-earth emission and rare-earth concentration dependences of NC-related emission indicate a certain concentration-dependent energy exchange between metal oxide NCs and rare-earth ions in silica matrix that cannot be simply explained as the changes of surrounding environment of Er3+ ions.We ascribed this energy exchange process to Forster resonance energy transfer(FRET),which only occurs when donor and acceptor are close to a critical distance as?2nm.The well spectral overlap between metal oxide NC surface-defect-related emission and rare-earth ion characteristic absorption is the precondition to allow efficient FRET.Among the 5D0-7Fj transitions of Eu3+ ions,the 5D0-7F2 band at 613 nm is an electric dipole transition and sensitive to chemical bonds in the vicinity of Eu3+ ion.On the other hand,the 5D0-7F1 band at 590 nm is a magnetic dipole one and hardly varies with the crystal field strength around the Eu3+ ion.Therefore,the fluorescence intensity ratio of 5D0-7F2 to 5D0-7F1 transitions,called as the asymmetry ratio,gives a measure of the degree of distortion from the inversion symmetry of the local environment of the Eu3+ion in matrix.In our photoluminescence spectra results of Eu3+ and SnO2 NCs co-doped silica sample,the dominant emission at 613 nm was observed gradually replaced by emission at 590 nm with increasing Sn concentration.At the same time,The 590 nm emission split into three sharp peaks.The decrease of asymmetry ratio implied that Eu3+ ions partially incorporated into SnO2 NCs for strong 590 nm emission only occurs where the surrounding of Eu3+ ions has high spatial symmetry.The incorporation of Eu3+ ions into SnO2 NCs shortened the average distance between them,leading to higher energy transfer efficiency.The similar phenomenon was not observed in Eu3+ ions and In2O3 NCs co-doped sample indicating different solubility of rare-earth ions in these materials.At last,we measured the temperature-dependent PL spectra and PL lifetimes of metal oxide NCs and rare-earth ions co-doped silica sample,on the purpose of evaluating the energy transfer efficiencies from NCs to rare-earth ions.The results turned out that,energy transfer efficiency from SnO2 NC to Er3+ ion was up to 63%in optimal situation which is higher than?20%of In2O3 NC to Er3+ ion.These may be due to the higher annealing temperature SnO2 NC can overcome,and partially incorporation of rare-earth ions into SnO2 NCs.In the works referred above,we demonstrated the synthesis of high-quality colloidal silicon NCs with efficient and color-tunable emission.Then we fabricated high-concentrated silicon nano-ink and a photodiode device with well UV light response from it.We also constructed metal oxide NCs and rare-earth ions co-doped silica thin films showing strongly enhanced rare-earth characteristic red and infrared emission.All these results indicate that the obtained silicon-based luminescent materials have high potentials in fabrications of high-performance optoelectronic devices,like laser sources on chips,planar waveguides and amplifiers,solution-processing solar cells and photodetectors. |
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