Thiol-Capping of CdTe Nanocrystals:An Alternative to Organometallic Synthetic Routes Nikolai Gaponik,*Dmitri V.Talapin,Andrey L.Rogach,*Kathrin Hoppe,
Elena V.Shevchenko,Andreas Kornowski,Alexander Eychmu1ller,and Horst Weller
Institute of Physical Chemistry,Uni V ersity of Hamburg,Bundesstr.45,20146Hamburg,Germany
Recei V ed:January24,2002;In Final Form:March28,2002
New approaches to synthesize photostable thiol-capped CdTe nanocrystals are reported.Post-preparative size-
selective precipitation and selective photochemical etching have been developed as methods providing an
increase of photoluminescence quantum efficiency of the nanocrystals of up to40%.Some advantages of
thiol-capping in comparison to conventional organometallic syntheses of quantum dots are discussed.
1.Introduction
Semiconductor nanocrystals,or colloidal quantum dots(QDs), show unique size-dependent optical properties1and are currently of great interest for applications in optoelectronic2-4and photovoltaic5,6devices,optical amplifier media for telecom-munication networks,7,8and for biolabeling.9-12The achieve-ment of desired particle sizes over the largest possible range, narrow size distribution,good crystallinity,photostability, desired surface properties,and high luminescence quantum yields are the parameters that are considered to be characteristics of a“high quality”of the chemically prepared semiconductor QDs.
In the framework of the colloidal chemistry approach,high quality nanocrystals of different II-VI and III-V semiconductor materials can now be obtained.The two existent general strategies of nanocrystal preparations are an organometallic synthesis based on the high-temperature thermolysis of the precursors13-19or on a dehalosilylation reaction,20,21and the synthesis in an aqueous medium using polyphosphates22or thiols23-27as stabilizing agents.CdTe nanocrystals are the substance providing very high photoluminescence quantum efficiencies:the value of65%at room temperature has been reported for organometallically prepared CdTe QDs.17,28How-ever,potential applications of these nanocrystals are hindered due to an instability of their photoluminescence in air.17,28On the other hand,since the appearance of our first report on the aqueous synthesis of mercaptoethanol-and thioglycerol-capped CdTe nanocrystals in1996,24sufficient progress has been made in the preparation and the design of the surface properties of thiol-capped CdTe nanocrystals whose luminescence is very stable and covers almost the whole visible spectral range(500-730nm)depending on the particle size,as well as in their characterization29,30and their use for numerous applications. Worth mentioning is the utilization of luminescent thiol-capped CdTe QDs in light-emitting devices(LEDs),4,31,32photonic33-36 and core-shell structures,35,37,38and as biological labels.39 Recent investigations have shown the attractivity of this material for light energy conversion.40In this article we report on the aqueous synthesis(Section3.1),the structure(Section3.2), optical properties,and processability of thiol-capped CdTe nanocrystals,with an emphasis on the procedures leading to highly luminescent(room-temperature quantum efficiencies up to40%)particles(Sections3.3and3.4).We also discuss the advantages and disadvantages of different thiols used as capping agents of CdTe QDs which are relevant for the further design of surface properties of the nanocrystals,and stress some advantages of the aqueous synthesis in comparison to the organometallic routes,e.g.,conventional TOPO-TOP(trioctyl-phosphine oxide-trioctylphosphine)approach(Section3.5). Summarizing the experimental data available today,thiol-capped CdTe nanocrystals synthesized in aqueous solution represent a kind of stable highly luminescent core-shell QD system with a naturally sulfur-capped surface(CdS shell)created by mer-capto-groups covalently attached to the surface cadmium atoms.
2.Experimental Section
All chemicals used were of analytical grade or of the highest purity available.Al2Te3(lumps)used as a source of H2Te was purchased from CERAC Inc.,https://www.doczj.com/doc/072826054.html,li-Q water(Millipore) was used as a solvent.
UV-vis absorption spectra were recorded with a Cary50 spectrophotometer(Varian).Photoluminescence(PL)measure-ments were performed at room temperature using a FluoroMax-2 spectrofluorimeter(Instruments SA).The room-temperature PL quantum efficiency(QE)of CdTe nanocrystals was estimated following the procedure of ref41by comparison with Rhodamine 6G(laser grade,Lambda Physik)in ethanol(Uvasol)assuming its PL QE as95%.42High-resolution transmission electron microscopy(HRTEM)and energy-dispersive X-ray analysis (EDX)were performed on a Philips CM-300microscope operating at300kV.TEM samples were prepared by dropping diluted solutions of transferred in toluene(by surface exchange of capping agent with1-dodecanethiol)CdTe nanocrystals onto 400-mesh carbon-coated copper grids with the excessive solvent immediately evaporated.Powder X-ray diffraction(XRD) measurements were carried out with a Philips X′Pert diffrac-tometer(Cu K R-radiation,variable entrance slit,Bragg-Brentano geometry,secondary monochromator).Samples for this study were prepared by placing finely dispersed powders of CdTe nanocrystals on standard Si supports.
To investigate the photostability of CdTe QDs,dilute colloidal solutions were irradiated for different time intervals with light of a450W xenon lamp cut around400nm by a band-pass filter having a bandwidth of～40nm and a peak transmission of～40%.
*Corresponding authors.Fax:+49-40-428383452.E-mail addresses: gaponik@chemie.uni-hamburg.de.rogach@chemie.uni-hamburg.de.7177
J.Phys.Chem.B2002,106,7177-7185
10.1021/jp025541k CCC:$22.00?2002American Chemical Society
Published on Web06/21/2002
3.Results and Discussion
3.1.Synthesis of Thiol-Stabilized CdTe Nanocrystals.In a typical synthesis (Figure 1)0.985g (2.35mmol)of Cd(ClO 4)2?6H 2O is dissolved in 125mL of water,and 5.7mmol of the thiol stabilizer (Table 1shows mercapto-compounds typically used as capping agents)are added under stirring,followed by adjusting the pH to the appropriate values (depending on the stabilizer nature,see Table 1)by dropwise addition of 1M solution of NaOH.Occasionally,the solution can remain slightly turbid at this stage because of the incomplete solubility of Cd thiolate complexes,but this does not influence the further synthesis.The solution is placed in a three-necked flask fitted with a septum and valves and is deaerated by N 2bubbling for ～30min.Under stirring,H 2Te gas (generated by the reaction of 0.2g (0.46mmol)of Al 2Te 3lumps with 15-20mL of 0.5M H 2SO 4under N 2atmosphere)is passed through the solution together with a slow nitrogen flow for ～20min.CdTe precursors are formed at this stage which is accompanied by a change of the solution color,depending on the thiols used,to yellow (1-thioglycerol,2-mercaptoethanol),orange (thioglycolic acid),or dark-red (2-(dimethylamino)ethanethiol,2-mercapto-ethylamine).They show an absorption spectrum being unstruc-tured in the visible spectral region with a tail extending to 650-700nm,and no luminescence.The precursors are converted to
Figure 1.Schematic presentation of the synthesis of thiol-capped CdTe QDs.First stage:formation of CdTe precursors by introducing H 2Te gas.Second stage:formation and growth of CdTe nanocrystals promoted by reflux.
TABLE 1:Overview of the Conditions Used for the Aqueous Synthesis of CdTe Nanocrystals,Their Properties,and Additional Comments on Their Use
stabilizer
pH used for the synthesis stability a of CdTe QDs surface charge b of CdTe QDs typical PL QE of as-prepared CdTe QDs
additional comments
2-mercaptoethanol 11.2-11.8stable slightly negative in alkaline <1%“Magic”clusters of the (supposed)formula Cd 54Te 32(SCH 2CH 2OH)328-are formed 291-thioglycerol
11.2-11.8
stable
slightly negative in alkaline 3%
Have been used for synthesis of highly luminescent CdHgTe QDs 27and for electrophoretic deposition of closely packed films 43
mixture (1:1)of
1-thioglycerol and
2,3-dimercapto-1-propanol 11.2-11.8moderate slightly negative in alkaline 6%High affinity to different surfaces
(gold,glass,latex,polymers,etc.)35,37thioglycolic acid (TGA)
11.2-11.8
stable
negative
10%
Have been used for fabrication of LEDs and graded films by layer-by-layer assembly 4,44,45and for electrophoretic deposition into pores of artificial opals.36Can potentially be used for bio-conjugation and for the synthesis of QDs conjugates.2-mercaptoethylamine (MA) 5.6-5.9moderate positive 10%
Have been used for fabrication of
latex core -semiconductor shell structures.37Can potentially be used for bio-conjugation and for the synthesis of QDs conjugates.L -cysteine 11.2-11.8moderate
negative or positive depending on the pH 10%
Have been used for synthesis of conjugates with bovine serum albumin.392-(dimethylamino)ethanethiol
5.0-
6.0moderate
positive
30%
Under investigation.
a
“Stable”means here that colloidal solutions of CdTe QDs are stable for months and even years being stored under air in the dark at room temperature.“Moderate”means that colloidal solutions coagulate occasionally during the storage;however,they are generally stable for months as well.Independent of the stabilizer nature,CdTe QDs are generally stable (no oxidation,no or only very minor changes of the optical properties)for years in powder form and in closely packed or nanocrystal/polymer films being kept in the dark under air.b The charge of nanocrystals was evaluated from -potential measurements with employment of oppositely charged latex beads.The experimental conditions were chosen so that the -potential of the beads changed its sign after adsorption of a monolayer of nanoparticles.
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CdTe nanocrystals by refluxing the reaction mixture at 100°C under open-air conditions with condenser attached (Figure 1).A clearly resolved absorption maximum of the first electronic transition of CdTe QDs appears at ～420nm in 5-10min after beginning of reflux corresponding to the smallest CdTe nanoc-rystals (<2nm size)which shifts to longer wavelengths as the particles grow in the course of heating.Green (luminescence maximum at ～510nm)band-edge emission appears in 10-15min after beginning of reflux when the CdTe nanocrystals reach the size of ～2nm.The size of the CdTe QDs growing further is controlled by the duration of reflux and can easily be monitored by absorption and PL spectra.The duration of the heat treatment necessary to reach a certain particle size depends on the nature of the stabilizer.Thus,it takes 2-3days to grow ～5nm large CdTe nanocrystals (luminescence maximum at ～650nm)in the presence of thioglycolic acid or mercaptoet-hylamine and up to 12days when thioglycerol is used as a stabilizer.The same tendency was observed for the growth of CdSe nanocrystals stabilized with thio acids or thioalcohols.25To grow larger CdTe nanocrystals with the emission spreading into the near-IR (PL maximum up to 730nm)additional injections of precursors are applied.
Note,that the synthesis of thiol-capped CdTe nanocrystals can also be done in a nonaqueous (dimethylformamide,DMF)solution by reaction of cadmium lactate with H 2Te gas in the presence of thioglycerol as a stabilizer.43Because of the higher boiling point of DMF (130°C)the particle growth proceeds much faster than in aqueous solution:it takes only 2h to produce CdTe nanocrystals emitting at 650nm.CdTe nanoc-rystals synthesized in DMF might be of special interest for forming composites with polymers soluble in organics.
Figure 2shows typical absorption and room-temperature PL spectra of a size series of CdTe nanocrystals.The spectra were measured on as-prepared CdTe colloidal solutions which were taken from the refluxing reaction mixture at different intervals of time and diluted with water to provide the optical densities
appropriate for PL measurements (about 0.1at the excitation wavelength).All samples show a well-resolved absorption maximum of the first electronic transition indicating a suf-ficiently narrow size distribution of the CdTe QDs,which shifts to the longer wavelengths with increasing size of the nano-crystals as a consequence of the quantum confinement.The room-temperature PL excitation spectra (Figure 3)also display electronic transitions at higher energies.PLE technique allows detection of the luminescence emitted by particles with selected size.The difference between PLE and absorption spectra is a result of the inhomogeneous broadening which affects the UV -vis spectra to a greater degree than PLE ones.The PL bands (Figure 2)are located close to the absorption thresholds (so-called band-edge or ”excitonic”photoluminescence)and are sufficiently narrow (full width at half-maximum,fwhm,as low as 35nm being increased up to 55-60nm for size fractions of large CdTe nanocrystals).The position of the PL maximum of the smallest (～2nm)luminescing CdTe QDs is located at 510nm (green emission),whereas the largest (～6nm)CdTe nanocrystals obtained emit in the near-IR with a PL maximum at 730nm.The whole spectral range between these two wavelengths is covered by the intermediate sizes of CdTe QDs.The PL QE of as-synthesized CdTe nanocrystals depends on the nature of the stabilizing agent (Table 1)and lies typically between 3and 10%,although values of 30-35%for 2-(dim-ethylamino)ethanethiol-stabilized nanocrystals were attained.In most cases it can be sufficiently improved by post-preparative treatments of the nanocrystals as will be shown below.
Table 1provides an overview of different thiols used by us as capping agents for CdTe nanocrystals.As already mentioned above,the nature of the thiol influences the particle growth and the PL QE of as-synthesized CdTe nanocrystals.Besides that,each type of stabilizer has its own advantages and disadvantages allowing the use of CdTe nanocrystals capped by varying thiols for different purposes.This is also briefly summarized in Table 1.CdTe QDs stabilized by thioglycolic acid (TGA)or by mercaptoethylamine (MA)show efficient luminescence as prepared and possess either negative or positive surface charge because of the surface carboxylic or amino groups,
respectively.
Figure 2.Absorption and PL spectra (λex
)400nm)of CdTe nanocrystals taken as prepared.The stabilizers used were 2-mercap-toethanol,2-(dimethylamino)ethanethiol,or thioglycolic acid.The smallest nanoparticles stabilized by 2-mercaptoethanol possess only a weak broad emission,associated with surface traps (not shown in the figure).Nanocrystals with emission maxima above 670nm were obtained by additional injections of Cd and Te precursors.
Figure 3.PL excitation spectra of TGA-capped CdTe nanocrystals showing well-resolved maxima of high-energy electronic transitions.The emission wavelengths are indicated by arrows.
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The possibility to manipulate these nanocrystals,e.g.,by the layer-by-layer technique4,44,45or by electrophoresis,36,43,45and to use the free functional groups of the capping molecules for conjugation39makes them especially attractive for fabrication of functional materials.In the following sections,we mainly discuss the TGA-and MA-capped CdTe nanocrystals and describe ways of further improvement of their photolumines-cence efficiency.
3.2.Structural Characterization of Thiol-Capped CdTe Nanocrystals.Figure4shows XRD patterns obtained from powdered precipitated fractions of CdTe nanocrystals synthe-sized at different conditions.The nanocrystals belong to the cubic(zinc blende)structure which is also the dominant crystal phase of bulk CdTe.However,the positions of the XRD reflexes of CdTe QDs synthesized under prolonged refluxing in the presence of thioglycolic acid or thioglycerol(Figure4)are intermediate between the values of the cubic CdTe and the cubic CdS phases.As discussed in ref43,prolonged refluxing of the aqueous colloidal solutions of CdTe nanocrystals in the presence of an excess of thiols in basic media leads to partial hydrolysis of the thiols and to the incorporation of the sulfur from the thiol molecules into the growing nanoparticles.Mixed CdTe(S)QDs, most probably with some gradient of sulfur distribution from inside the nanocrystals to the surface,are formed under these conditions as evidenced from the temporal evolution of the XRD patterns during refluxing(Figure4a).Indeed,the smaller nanocrystals synthesized at moderate conditions(short refluxing time)show only slight deviations of the XRD reflexes from CdTe zinc blende diffraction patterns.The use of DMF instead of water as a solvent allows the prevention of the hydrolysis of thiols and,thus,diminishes the incorporation of sulfur into largely grown nanocrystals.43The aqueous synthesis performed at comparatively low pH values(5.6-5.9)in the presence of 2-mercaptoethylamine as a stabilizer leads to the formation of CdTe nanocrystals whose XRD patterns show only slight traces of a CdS phase(Figure4b).
HRTEM investigations of CdTe nanocrystals grown at different conditions confirmed the XRD data.The longer the time of refluxing in the basic medium,the larger was the sulfur content in the mixed CdTe(S)nanocrystals as indicated by the lattice plane distances in the HRTEM images being intermediate between cubic CdTe and cubic CdS phases.43In contrast,MA-capped CdTe nanocrystals synthesized at slightly acidic pH showed the lattice plane distances of the cubic CdTe phase.As is evident from Figure5the thiol-capped CdTe nanoparticles are crystalline,sufficiently monodisperse and well separated, with a mean size of～4.5nm which is in good accordance with the size calculated from the XRD data by the Scherrer equation. Thus,both XRD and TEM investigations show that crystalline (cubic zinc blende structure)particles of either pure CdTe or mixed CdTe(S)phase are formed in the aqueous synthesis depending on the reaction conditions(pH value,duration of reflux,nature of stabilizer).
Worth mentioning is the temperature-dependent EXAFS study carried out on2-mercaptoethanol capped CdTe nanocrystals and reported in detail in ref29.The investigation allowed the separation of the structural and dynamic properties of the CdTe core and the Cd-SR shell.The formula[Cd54Te32(SCH2-CH2OH)52]8-was suggested for these extremely small clusters consisting of a tetrahedral CdTe zinc blende core partially coated by a Cd-SR surface layer.
3.3.Photostability of Thiol-Capped CdTe Nanocrystals. As indicated in Table1,colloidal solutions of CdTe QDs are stable and do not change their optical properties for months and even years(depending on the nature of the stabilizer)being kept under air in the dark at room temperature.Light sensitivity (photochemical stability)of aqueous CdTe colloids was found to be strongly dependent on the presence of oxygen and free stabilizer molecules in solution and can be greatly improved by keeping them under inert atmosphere.We have compared the room-temperature photostability of aqueous colloids of TGA-capped and MA-capped CdTe nanocrystals with the photo-stability of organometallically synthesized CdTe QDs and TOPO-capped CdSe nanocrystals.The latter QDs were reported to be stable enough for,e.g.,applications in light-emitting devices.3,46
The optical densities of irradiated solutions of TGA-capped CdTe QDs in water and TOPO-capped CdSe QDs in toluene were0.15at400nm.The absorption and the PL spectra of CdTe and CdSe nanocrystals were checked after different intervals of irradiation time.The long-term photostability of deaerated solutions of the both materials was found to be comparable and very high.The photodegradation of TGA-capped CdTe QDs in oxygen-saturated solutions proceeded approximately20times faster than under airless conditions. However,taking into account the rigid conditions of illumination their photostability even in this case was very high in compari-son with organometallically synthesized CdTe QDs17and was comparable with the stability of TOPO-capped CdSe nano-crystals(Figure6).
The mechanism of nanocrystal photodegradation seems to be different in the cases of TGA-capped CdTe and TOPO-capped CdSe nanocrystals.Both in airless and in oxygen-
Figure4.Temporal evolution of powder X-ray diffractograms during the synthesis of thioglycerol-capped CdTe nanocrystals(a).X-ray diffractograms of CdTe nanocrystals obtained in aqueous solutions in the presence of TGA and MA(b).Figure5.TEM overview and HRTEM(inset)images of TGA-capped CdTe nanocrystals.The particles were transferred from water to toluene in the presence of1-dodecanethiol in order to achieve their better separation on the TEM grids.
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saturated solutions the illumination of CdTe nanocrystals leads first to a sufficient improvement of their PL QE,followed by a gradual quenching of PL intensity and finally to coagulation of the colloid.The nanocrystals in the precipitate normally still luminesce.In the case of TOPO-capped CdSe nanocrystals the PL QE slowly goes down during the illumination until the luminescence disappeared completely,while no coagulation is observed (Figure 6).
The photostability of MA-capped CdTe nanocrystals in aerated aqueous solutions was found to be lower compared to that of the TGA-capped CdTe QDs and no improvement of the PL QE was observed at initial stages of irradiation.The difference between the behavior of TGA-and MA-stabilized CdTe QDs originates most probably from their structural difference.The TGA-capped nanocrystals are essentially in the mixed CdTe(S)phase with sulfur-enriched pre-surface layers,whereas MA-capped particles are almost pure CdTe.The irradiation promotes oxidation of unsaturated Te atoms which were recently identified as hole traps by optically detected magnetic resonance 47and are known to be highly susceptible to oxidation.48This treatment leads to an improvement of the CdS shell around the CdTe core via substitution of oxidized surface Te sites by sulfur from the stabilizer molecules.The formation of such a shell enhances the PL efficiency of TGA-capped CdTe QDs at the initial stages of illumination and greatly inhibits the following photodegradation of the nanocrystals.In fact,an enhancement of the PL efficiency at initial stages of irradiation was also observed for CdSe/ZnS core -shell nano-crystals.49In the case of MA-stabilized CdTe QDs where sulfur atoms are hardly present in the crystalline structure,a complete outer CdS shell cannot be formed at the expense of thiol molecules of the stabilizing shell resulting in their faster photodegradation.The absence of a stabilizing CdS shell in the case of organometallically synthesized CdTe nanocrystals whose surface is capped by TOP and amines 17causes a very quick loss of the photoluminescence in air as well.
In addition to the above-mentioned processes,a photocatalytic oxidation of thiol molecules on the surface of nanocrystals can also take place as was shown for thiol-capped CdSe nanocrys-tals.50The thiol ligands on the surface of the QDs convert into disulfides under irradiation leading to precipitation of the nanocrystals if no new thiol ligands are available in solution.This model correlates well with the behavior of the TGA-capped
CdTe nanocrystals allowing the explanation of their higher photostability in the presence of the excess of free thiol stabilizer in solution.51
The photostability of all kinds of thiol-capped CdTe nano-crystals in the solid state (powders,closely packed QD films,nanocrystal/polymer composites)is comparable,very high,and independent of the nature of the stabilizer.Thus,a month of storage under daylight and air did not cause recognizable changes of the optical properties of spin-coated or casted closely packed films of both TGA-and MA-stabilized nanocrystals.The layer-by-layer formed films of TGA-capped CdTe QDs with poly(diallyldimethylammonium chloride)as a polyelectrolyte also remained highly luminescent under the same conditions,but a slight bleaching was observed within weeks.
3.4.Post-Preparative Improvement of the Photolumines-cence Efficiency of Thiol-Capped CdTe Nanocrystals.Taking into account the growing demand on strongly luminescing semiconductor nanocrystals for light-emitting devices 2-4,46and tagging applications,9-12several strategies of improvement of the PL efficiency of thiol-capped CdTe nanocrystals have been developed.The earliest attempt was the study of the pH dependence on the luminescence efficiency of TGA-capped CdTe QDs.51It was found that the PL QE can be enhanced up to 5times (from 3-4%being characteristic for the TGA-capped CdTe QDs synthesized by using a NaHTe solution as the tellurium source to 18-20%)by decreasing the pH of the colloidal solution to 4.5-5.0in the presence of excessive thioglycolic acid.At these conditions,a shell of thiolate complexes is probably formed at the surface of the CdTe nanocrystals acting as a wide-bandgap material analogous to,for example,a CdS 52or ZnS shell 16,53on CdSe cores and eliminating the surface traps which are centers of nonradiative recombination.
Further studies have shown the possibility of a sufficiently larger improvement of the PL efficiency (up to 40%at room temperature)of thiol-capped CdTe nanocrystals.Importantly,simple methods of the post-preparative size-selective precipita-tion and selective photochemical etching have been found to work well,yielding highly luminescent CdTe nanocrystal fractions.Both methods are discussed below in more detail.3.4.1.Size-Selecti V e Precipitation.The post-preparative size-selective precipitation procedure 13,54can successfully be applied for isolation of thiol-capped CdTe nanocrystals from their crude solutions and for narrowing down their size distribution.The method is based on the gradual precipitation of the nanocrystals induced by addition of a nonsolvent (the largest particles precipitate first)allowing the separation of the initial colloidal solution into several fractions of nanocrystals with narrowed size distributions.In the case of thiol-capped CdTe nanocrystals,the procedure is carried out under air and is typically as follows.A portion of as-prepared colloidal solution containing CdTe nanocrystals of a certain mean size is concentrated ～5times using a rotary evaporator.Then,2-propanol is added dropwise under stirring until the solution becomes slightly turbid.The turbid dispersion is left stirring for further 15min,and the precipitate containing the first fraction of CdTe nanocrystals is isolated from the supernatant by centrifugation.Another portion of 2-propanol is added dropwise to the supernatant to obtain the second precipitated fraction of CdTe nanocrystals and so on.This procedure can be repeated several times to obtain up to 10-12size-selected fractions of CdTe nanocrystals from the initial solution.The precipitated nanocrystal fractions can be redissolved in water or dried and kept during a long period of time as water-soluble powders.
Figure 6.Evolution of the relative quantum efficiency of photo-luminescence of oxygen-saturated solutions of TGA-capped CdTe and TOPO-capped CdSe QDs under irradiation with 400nm light of a 450W xenon lamp.
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Importantly,no changes in the optical properties of the thiol-capped CdTe nanocrystals are induced by their size-selective precipitation.Thus,a comparison of the initial crude solution of TGA-capped CdTe QDs with a solution prepared by mixing all size-selectively isolated fractions in the same proportions
as they were presented in the crude solution shows nearly quantitative agreement between their UV -vis and PL spectra (Figure 7).This is probably a result of the strong capping of CdTe nanocrystals with thiol molecules during the post-preparative treatment.In contrast,in the case of organometal-lically synthesized CdSe or InAs nanocrystals a systematic decrease of the PL QE of size-selected fractions in comparison with their initial crude solutions was observed due to the partial removal of the more labile TOPO-stabilizing shells under conditions when the concentration of the excessive TOP in the solution was low.55
Figure 8shows absorption and PL spectra of the size-selected fractions obtained from two different portions of TGA-capped CdTe nanocrystals.The portions were taken from the same crude solution of the grown nanoparticles after different refluxing times,and thus contained CdTe nanocrystals of different mean sizes.The particle size distribution in each size-selected fraction is,as expected,narrower than in the initial crude solution leading to a better pronounced electronic transition in the absorption spectra and to narrower PL bands (fwhm 38-52nm,in initial solutions ～46-56nm).
There is a clear dependence of the efficiency of the band-edge PL on the “fraction number”of CdTe nanocrystals (Figure 8).The increase of the PL intensity is nonmonotonic:the maximum of the PL efficiency is always achieved for a fraction lying in the middle of the size-selected series,independent of
Figure 7.Absorption and PL spectra of an as-prepared crude solution of CdTe nanocrystals (solid line)and of a solution obtained by re-mixing all size-selected fractions isolated from this crude solution (dashed
line).
Figure 8.Absorption (a,
c)and PL (b,d)spectra of size-selected fractions isolated from two portions of a crude solution of CdTe nanocrystals taken after refluxing for 2.5h (a,b,)and 30h (c,d),respectively.The absorption and PL spectra of as-prepared crude solutions (bold lines)are added for comparison.PL intensities are normalized to identical absorbance at the excitation wavelength (400nm).
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the mean particle size of the nanocrystals in the initial crude solution.The difference between the intensities of the fractions of CdTe QDs with the lowest and the highest PL efficiency (referred in Figures8and9as LPL and HPL fractions, respectively)reaches about an order of magnitude,with a PL QE of25-30%for the HPL fractions.In addition,the photostability of the HPL fractions is higher than the photo-stability of any other fraction obtained from the same portion of the crude solution(Figure9).
The possibility to divide any crude solution into fractions of nanocrystals differing in their photoluminescence intensity and photostability was found by us for TOPO-capped CdSe and TOP-capped InAs QDs as well,55which indicates a general character of this phenomenon for any ensemble of colloidal semiconductor nanocrystals.The nonmonotonic distribution of PL intensities within the ensembles of QDs is inherent to the particle growth via an Ostwald ripening mechanism and originates,most probably,from different degrees of the nano-crystal surface disorder.Detailed investigation of this phenom-enon is presented elsewhere.55
Thus,highly luminescent fractions of thiol-capped CdTe nanocrystals with PL QE of20-30%are present in any portion of the crude solution and can be easily extracted by the size-selective precipitation technique.Furthermore,the PL efficiency of the HPL fractions of TGA-capped CdTe nanocrystals can be further enhanced up to40%by the formation of a shell of cadmium thiolate complexes at the surface of the nanoparticles using the method of ref51.
3.4.2.Selecti V e Photochemical Etching.As mentioned in Section3.3,the photoluminescence efficiency of TGA-capped CdTe nanocrystals increases at the initial stages of photooxi-dation,presumably due to the etching of tellurium trap states and an improvement of the CdS shell.Further investigations of photochemical etching showed the possibility of using this technique to enhance the luminescence of CdTe nanocrystals. The etching was carried out by illumination with a450W xenon lamp under aerobic conditions.The high-energy part of the lamp spectrum was cut off by optical filters in order to illuminate the samples near the absorption edges.Under these conditions an exposure for5days typically leads to both a～3-fold improvement of the PL QE(up to30%)and narrowing the PL band of the crude solutions of TGA-stabilized CdTe QDs(Figure 10).Surprisingly,the positions of the PL maxima of the highly luminescent fractions obtained by photochemical etching co-incide almost exactly((1nm)with those of the size-selectively precipitated HPL fractions when the same crude solutions are taken.The photochemical etching under polychromatic light leads to an increase of the PL QE up to40%coming along with a broadening of the photoluminescence and absorption bands owing to different rates of etching of different nano-crystals in the ensemble.We suggest that the improvement of the PL QE is due to the eliminating of defect states(or carrier traps)s most probably unsaturated surface tellurium atoms.47The nanocrystals possessing the most defect states dissolve primarily during photochemical etching.On the other hand the elimination of a few(or single)Te traps does not lead to the degradation of the low-defect QDs due to the passivating role of their CdS shell.Moreover,the vacancies formed can be successfully saturated by an excess of thiol molecules being available in solution.
The fractions of etched CdTe nanocrystals with PL QE of ～40%show a temporal stability against coagulation comparable with the initial solutions of the nanocrystals.The luminescence properties of the former as solid films(both casted and spin-coated)were found to be stable for at least4weeks under daylight conditions.
3.5.Aqueous Synthesis vs Organometallic Approach: Advantages and Disadvantages.In this section we discuss some general advantages of the aqueous synthesis and compare some characteristics of thiol-capped CdTe nanocrystals with corresponding data of organometallically synthesized QDs.The comparison with organometallically synthesized QDs will be done based both on their properties published in the literature and on our own experiences in the synthesis and handling of CdTe,17CdSe,16,55,56InP,57InAs,55and CdSe/ZnS16nanocrystals. Among the advantages of the aqueous synthesis its simplicity and high reproducibility should be mentioned.Thus,colloids of CdTe nanocrystals with a particle size distribution about10% can be prepared without the hot-injection technique which is necessary for the synthesis of nanocrystals in TOPO-TOP mixture(e.g.,CdSe,13CdTe,28InAs58).As a result,the aqueous synthesis of thiol-capped nanocrystals can be carried out equally effectively on a vast scale,whereas the scaling up of the organometallic synthesis is difficult59due to its poor reproduc-ibility15,18and the use of extremely dangerous reactants(e.g., dimethylcadmium).The aqueous synthesis was successfully carried out by us yielding up to10g of CdTe nanocrystals per synthesis and further scaling up is possible.These nanocrystals
Figure9.The temporal evolution of the absorbance at400nm of the LPL(O)and the HPL(9)fractions of CdTe nanocrystals under irradiation with400nm light of a450W xenon lamp in oxygen-saturated solutions.The absorbance is normalized to the initial value (i.e.,before irradiation).Figure10.Evolution of absorption(left)and PL(right)spectra of TGA-stabilized CdTe nanocrystals exposed to selective etching at the low energy side of the absorption spectrum.The PL spectra are normalized to the absorption at the excitation wavelength(450nm).
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can be precipitated,washed and kept in the dry state under ambient conditions as long as two years being stable and resoluble in water.
Keeping in mind the potential importance of highly lumi-nescent nanocrystals for large-scale applications,we also estimated the cost of equimolar amounts(with respect to formula unit,i.e.,CdTe and CdSe)of TGA-capped CdTe and TOPO-capped CdSe nanocrystals whose luminescence efficiencies and photostabilities are similar.The costs of chemicals were compared using Aldrich’2001prices for most of the chemicals, Strem’2001price for dimethylcadmium and CERAC Inc.’2001 price for Al2Te3.The synthesis of CdSe QDs is about8times more expensive.In the case of organometallically prepared core/ shell nanocrystals(e.g.,CdSe/ZnS and CdSe/CdS)this differ-ence reaches orders of magnitude.However,in recent papers by Peng et al.low cost and less hazardous precursors(CdO, cadmium acetate,etc.)and stabilizing mixtures based on fatty acids were applied for the synthesis of nonaqueous highly luminescent CdSe nanocrystals.18,19The use of these alternative synthetic routes might considerably decrease the cost of organometallically prepared luminescent QDs. Nanocrystals synthesized by the aqueous approach do not possess the degree of crystallinity of the organometallically prepared QDs,where high annealing temperatures(200-360°C)are used during the synthesis.A very effective separation of nucleation and growth stages achieved in the organometallic synthesis by the so-called hot-injection technique13allows to reach narrower size distributions of the nanocrystals in com-parison with those prepared in aqueous solutions.However,the aqueous approach generally allows the synthesis of smaller QDs, both CdTe and CdSe,25and the post-preparative size-selective precipitation procedure works more reproducible in the case of aqueous colloids in terms of retaining the luminescence proper-ties,as discussed in Section3.4.1.
The possibility of controlling the surface charge and other surface properties of thiol-capped QDs simply by the choice of the stabilizing mercapto-compound with appropriate free func-tional groups is definitely important,especially when water-soluble nanocrystals are needed,e.g.,for fluorescent tagging applications.We refer the reader to the paper12for the criticism on the reported routes to make TOPO-capped nanocrystals with CdSe cores water-soluble,pointing out that the stepwise procedure proposed in this paper yielded CdSe/ZnS core-shell QDs whose PL QE does not exceed10-20%.TGA-capped water-soluble CdTe nanocrystals with stable PL QE up to40% reported in the present study might be an useful alternative in this case.
4.Conclusions
Thiol-capped CdTe nanocrystals represent a kind of core-shell system with a naturally sulfur-capped surface(CdS shell) created by mercapto-groups covalently attached to the surface cadmium atoms.By reaction conditions facilitating the formation of mixed CdTe(S)nanocrystals,as mentioned above for the TGA-capped CdTe QDs,the sulfur-rich region is most probably stretched from the surface into the nanocrystal.Importantly,the synthesis of such kind of core-shell nanocrystals naturally occurs in one step,as the sulfur originates from the stabilizing thiol molecules during the particle growth.At the bulk CdTe/ CdS interface,the conduction band step,i.e.,the offset of the absolute band position,is close to zero,whereas the valence band step is～1eV as given in ref60.The wave functions calculated for a CdTe/CdS system with the particle-in-a-box model61show a delocalization of the electron through the entire structure and the confinement of the hole in the CdTe core s the same picture as reported for the CdSe/CdS nanocrystals providing the photostability and electronic accessibility.52The effective screening of the hole by the shell explains the high photostability of these nanocrystals,since the main photodeg-radation mechanism is the photooxidation of surface tellurium atoms requiring oxygen and holes.22,48The weak localization of electrons promotes the electronic accessibility of thiol-capped CdTe nanocrystals,allowing,e.g.,an effective electron injection important for LED applications.4,31,32The additional functional groups of the thiol capping molecules of the nanocrystals provides their water solubility,high processability,and surface charge desired.
Acknowledgment.We thank J.Kolny for assistance with powder X-ray diffraction measurements,and Prof.Dr.M.Gao for useful discussions.This work was supported in part by the DFG Schwerpunktprogramm“Photonic Crystals”,the BMBF-Philips research project,NATO Collaborative Linkage Grant CLG976365and Fond der Chemischen Industrie. References and Notes
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如何写先进个人事迹 篇一：如何写先进事迹材料 如何写先进事迹材料 一般有两种情况：一是先进个人，如先进工作者、优秀党员、劳动模范等；一是先进集体或先进单位，如先进党支部、先进车间或科室，抗洪抢险先进集体等。无论是先进个人还是先进集体，他们的先进事迹，内容各不相同，因此要整理材料，不可能固定一个模式。一般来说，可大体从以下方面进行整理。 (1)要拟定恰当的标题。先进事迹材料的标题，有两部分内容必不可少，一是要写明先进个人姓名和先进集体的名称，使人一眼便看出是哪个人或哪个集体、哪个单位的先进事迹。二是要概括标明先进事迹的主要内容或材料的用途。例如《王鬃同志端正党风的先进事迹》、《关于评选张鬃同志为全国新长征突击手的材料》、《关于评选鬃处党支部为省直机关先进党支部的材料》等。 (2)正文。正文的开头，要写明先进个人的简要情况，包括：姓名、性别、年龄、工作单位、职务、是否党团员等。此外，还要写明有关单位准备授予他(她)什么荣誉称号，或给予哪种形式的奖励。对先进集体、先进单位，要根据其先进事迹的主要内容，寥寥数语即应写明，不须用更多的文字。 然后，要写先进人物或先进集体的主要事迹。这部分内容是全篇材料
的主体，要下功夫写好，关键是要写得既具体，又不繁琐；既概括，又不抽象；既生动形象，又很实在。总之，就是要写得很有说服力，让人一看便可得出够得上先进的结论。比如，写一位端正党风先进人物的事迹材料，就应当着重写这位同志在发扬党的优良传统和作风方面都有哪些突出的先进事迹，在同不正之风作斗争中有哪些突出的表现。又如，写一位搞改革的先进人物的事迹材料，就应当着力写这位同志是从哪些方面进行改革的，已经取得了哪些突出的成果，特别是改革前后的．经济效益或社会效益都有了哪些明显的变化。在写这些先进事迹时，无论是先进个人还是先进集体的，都应选取那些具有代表性的具体事实来说明。必要时还可运用一些数字，以增强先进事迹材料的说服力。 为了使先进事迹的内容眉目清晰、更加条理化，在文字表述上还可分成若干自然段来写，特别是对那些涉及较多方面的先进事迹材料，采取这种写法尤为必要。如果将各方面内容材料都混在一起，是不易写明的。在分段写时，最好在每段之前根据内容标出小标题，或以明确的观点加以概括，使标题或观点与内容浑然一体。 最后，是先进事迹材料的署名。一般说，整理先进个人和先进集体的材料，都是以本级组织或上级组织的名义；是代表组织意见的。因此，材料整理完后，应经有关领导同志审定，以相应一级组织正式署名上报。这类材料不宜以个人名义署名。 写作典型经验材料-般包括以下几部分： (1)标题。有多种写法，通常是把典型经验高度集中地概括出来，一
How to manage time Time treats everyone fairly that we all have 24 hours per day. Some of us are capable to make good use of time while some find it hard to do so. Knowing how to manage them is essential in our life. Take myself as an example. When I was still a senior high student, I was fully occupied with my studies. Therefore, I hardly had spare time to have fun or develop my hobbies. But things were changed after I entered university. I got more free time than ever before. But ironically, I found it difficult to adjust this kind of brand-new school life and there was no such thing called time management on my mind. It was not until the second year that I realized I had wasted my whole year doing nothing. I could have taken up a Spanish course. I could have read ten books about the stories of successful people. I could have applied for a part-time job to earn some working experiences. B ut I didn’t spend my time on any of them. I felt guilty whenever I looked back to the moments that I just sat around doing nothing. It’s said that better late than never. At least I had the consciousness that I should stop wasting my time. Making up my mind is the first step for me to learn to manage my time. Next, I wrote a timetable, setting some targets that I had to finish each day. For instance, on Monday, I must read two pieces of news and review all the lessons that I have learnt on that day. By the way, the daily plan that I made was flexible. If there’s something unexpected that I had to finish first, I would reduce the time for resting or delay my target to the next day. Also, I would try to achieve those targets ahead of time that I planed so that I could reserve some more time to relax or do something out of my plan. At the beginning, it’s kind of difficult to s tick to the plan. But as time went by, having a plan for time in advance became a part of my life. At the same time, I gradually became a well-organized person. Now I’ve grasped the time management skill and I’m able to use my time efficiently.
英语演讲稿：未来的工作 这篇《英语演讲稿范文：未来的工作》，是特地，希望对大家有所帮助！ 热门演讲推荐：竞聘演讲稿 | 国旗下演讲稿 | 英语演讲稿 | 师德师风演讲稿 | 年会主持词 | 领导致辞 everybody good afternoon:. first of all thank the teacher gave me a story in my own future ideal job. everyone has a dream job. my dream is to bee a boss, own a pany. in order to achieve my dreams, i need to find a good job, to accumulate some experience and wealth, it is the necessary things of course, in the school good achievement and rich knowledge is also very important. good achievement and rich experience can let me work to make the right choice, have more opportunities and achievements. at the same time, munication is very important, because it determines whether my pany has a good future development. so i need to exercise their municative ability. i need to use all of the free time to learn
太阳能电池板与蓄电池配置计算公式(图) 太阳能电池板与蓄电池配置计算公式 一：首先计算出电流： 如：12V蓄电池系统； 30W的灯2只，共60瓦。 电流＝60W÷12V＝5A 二：计算出蓄电池容量需求： 如：路灯每夜累计照明时间需要为满负载7小时(h)； (如晚上8：00开启，夜11：30关闭1路，凌晨4：30开启2路，凌晨5：30关闭) 需要满足连续阴雨天5天的照明需求。(5天另加阴雨天前一夜的照明，计6天) 蓄电池＝5A×7h×(5＋1)天＝5A×42h＝210AH 另外为了防止蓄电池过充和过放，蓄电池一般充电到90%左右；放电余留20%左右。 所以210AH也只是应用中真正标准的70%左右。 三：计算出电池板的需求峰值(WP)： 路灯每夜累计照明时间需要为7小时(h)； ★：电池板平均每天接受有效光照时间为4.5小时(h)； 最少放宽对电池板需求20%的预留额。 WP÷17.4V＝(5A×7h×120%)÷4.5h WP÷17.4V＝9.33 WP＝162(W)
光伏发电系统计算方法 光伏系统的规模和应用形式各异，如系统规模跨度很大，小到几瓦的太阳能庭院灯，大到MW级的太阳能光伏电站。其应用形式也多种多样，在家用、交通、通信、空间应用等诸多领域都能得到广泛的应用。尽管光伏系统规模大小不一，但其组成结构和工作原理基本相同。 太阳能发电系统由太阳能电池组、太阳能控制器、蓄电池（组）组成。如输出电源为交流220V或11 0V，还需要配置逆变器。各部分的作用为： （一）太阳能电池板：太阳能电池板是太阳能发电系统中的核心部分，也是太阳能发电系统中价值最高的部分。其作用是将太阳的辐射能力转换为电能，或送往蓄电池中存储起来，或推动负载工作。 （二）太阳能控制器：太阳能控制器的作用是控制整个系统的工作状态，并对蓄电池起到过充电保护、过放电保护的作用。在温差较大的地方，合格的控制器还应具备温度补偿的功能。其他附加功能如光控开关、时控开关都应当是控制器的可选项； （三）蓄电池：一般为铅酸电池，小微型系统中，也可用镍氢电池、镍镉电池或锂电池。其作用是在有光照时将太阳能电池板所发出的电能储存起来，到需要的时候再释放出来。 （四）逆变器：在很多场合，都需要提供220VAC、110VAC的交流电源。由于太阳能的直接输出一般都是12VDC、24VDC、48VDC。为能向220VAC的电器提供电能，需要将太阳能发电系统所发出的直流电能转换成交流电能，因此需要使用DC-AC逆变器。在某些场合，需要使用多种电压的负载时，也要用到DC-DC逆变器，如将24VDC的电能转换成5VDC的电能（注意，不是简单的降压）。 光伏系统的设计包括两个方面：容量设计和硬件设计。 在进行光伏系统的设计之前，需要了解并获取一些进行计算和选择必需的基本数据：光伏系统现场的地理位置，包括地点、纬度、经度和海拔；该地区的气象资料，包括逐月的太阳能总辐射量、直接辐射量以及散射辐射量，年平均气温和最高、最低气温，最长连续阴雨天数，最大风速以及冰雹、降雪等特殊气象情况等。 蓄电池的设计包括蓄电池容量的设计计算和蓄电池组的串并联设计。首先，给出计算蓄电池容量的基本方法。 （1）基本公式
小学生个人读书事迹简介怎么写800字 书,是人类进步的阶梯,苏联作家高尔基的一句话道出了书的重要。书可谓是众多名人的“宠儿”。历来,名人说出关于书的名言数不胜数。今天小编在这给大家整理了小学生个人读书事迹,接下来随着小编一起来看看吧! 小学生个人读书事迹1 “万般皆下品,惟有读书高”、“书中自有颜如玉,书中自有黄金屋”,古往今来,读书的好处为人们所重视,有人“学而优则仕”,有人“满腹经纶”走上“传道授业解惑也”的道路……但是,从长远的角度看,笔者认为读书的好处在于增加了我们做事的成功率,改善了生活的质量。 三国时期的大将吕蒙,行伍出身,不重视文化的学习,行文时,常常要他人捉刀。经过主君孙权的劝导,吕蒙懂得了读书的重要性,从此手不释卷,成为了一代儒将,连东吴的智囊鲁肃都对他“刮目相待”。后来的事实证明,荆州之战的胜利,擒获“武圣”关羽,离不开吕蒙的“运筹帷幄,决胜千里”,而他的韬略离不开平时的读书。由此可见,一个人行事的成功率高低,与他的对读书,对知识的重视程度是密切相关的。 的物理学家牛顿曾近说过,“如果我比别人看得更远,那是因为我站在巨人的肩上”,鲜花和掌声面前,一代伟人没有迷失方向,自始至终对读书保持着热枕。牛顿的话语告诉我们,渊博的知识能让我们站在更高、更理性的角度来看问题,从而少犯错误,少走弯路。
读书的好处是显而易见的,但是,在社会发展日新月异的今天,依然不乏对读书,对知识缺乏认知的人,《今日说法》中我们反复看到农民工没有和用人单位签订劳动合同,最终讨薪无果;屠户不知道往牛肉里掺“巴西疯牛肉”是犯法的;某父母坚持“棍棒底下出孝子”,结果伤害了孩子的身心,也将自己送进了班房……对书本,对知识的零解读让他们付出了惨痛的代价,当他们奔波在讨薪的路上,当他们面对高墙电网时,幸福,从何谈起?高质量的生活,从何谈起? 读书,让我们体会到“锄禾日当午,汗滴禾下土”的艰辛;读书,让我们感知到“四海无闲田,农夫犹饿死”的无奈;读书,让我们感悟到“为报倾城随太守,西北望射天狼”的豪情壮志。 读书的好处在于提高了生活的质量,它填补了我们人生中的空白,让我们不至于在大好的年华里无所事事,从书本中,我们学会提炼出有用的信息,汲取成长所需的营养。所以,我们要认真读书,充分认识到读书对改善生活的重要意义,只有这样,才是一种负责任的生活态度。 小学生个人读书事迹2 所谓读一本好书就是交一个良师益友,但我认为读一本好书就是一次大冒险,大探究。一次体会书的过程,真的很有意思,咯咯的笑声,总是从书香里散发;沉思的目光也总是从书本里透露。是书给了我启示,是书填补了我无聊的夜空,也是书带我遨游整个古今中外。所以人活着就不能没有书,只要爱书你就是一个爱生活的人,只要爱书你就是一个大写的人,只要爱书你就是一个懂得珍惜与否的人。可真所谓
关于坚持的英语演讲稿 Results are not important, but they can persist for many years as a commemoration of. Many years ago, as a result of habits and overeating formed one of obesity, as well as indicators of overall physical disorders, so that affects my work and life. In friends to encourage and supervise, the participated in the team Now considered to have been more than three years, neither the fine rain, regardless of winter heat, a day out with 5:00 time. The beginning, have been discouraged, suffering, and disappointment, but in the end of the urging of friends, to re-get up, stand on the playground. 成绩并不重要，但可以作为坚持多年晨跑的一个纪念。多年前，由于庸懒习惯和暴饮暴食，形成了一身的肥胖，以及体检指标的全盘失常，以致于影响到了我的工作和生活。在好友的鼓励和督促下，参加了晨跑队伍。现在算来，已经三年多了，无论天晴下雨，不管寒冬酷暑，每天五点准时起来出门晨跑。开始时，也曾气馁过、痛苦过、失望过，但最后都在好友们的催促下，重新爬起来，站到了操场上。 In fact, I did not build big, nor strong muscles, not a sport-born people. Over the past few years to adhere to it, because I have a team behind, the strength of a strongteam here, very grateful to our team, for a long time, we encourage each other, and with sweat, enjoying common health happy. For example, Friends of the several run in order to maintain order and unable to attend the 10，000 meters race, and they are always concerned about the brothers and promptly inform the place and time, gives us confidence and courage. At the same time, also came on their own inner desire and pursuit for a good health, who wrote many of their own log in order to refuel for their own, and inspiring. 其实我没有高大身材，也没健壮肌肉，天生不属于运动型的人。几年来能够坚持下来，因为我的背后有一个团队，有着强大团队的力量，在这里，非常感谢我们的晨跑队，长期以来，我们相互鼓励着，一起流汗，共同享受着健康带来的快
太阳能电池计算 HEN system office room 【HEN16H-HENS2AHENS8Q8-HENH1688】
单晶硅太阳能电池板，铝合金边框，钢化玻璃面板 拍前请确认货期。 详细参数： 多晶硅太阳能板100W可充12V/24V 净重：11KGS 工作电压： 工作电流： 开路电压： 短路电流： 蓄电池：24V/12V 二、产品特点： 采用平均转换效率在15%以上的优质单晶硅太阳电池单片，具有优良的弱光响应性能，符合IEC61215和电气保护II级标准。太阳能电池转换效率高。而且太阳能电池板阵列一次性性能佳。 太阳能电池板阵列的表面采用高透光绒面钢化玻璃封装，气密性、耐候性好，抗腐蚀。 阳极氧化铝边框：机械强度高，具有良好的抗风性和防雹性，可在各种复杂恶劣的气候条件下使用，便于安装。 太阳能电池板在制造时，先进行化学处理，表面做成了一个象金字塔一样的绒面，能减少反射，更好地吸收光能。 采用双栅线，使组件的封装的可靠性更高。 太阳能电池板阵列抗冲击性能佳，符合IEC国际标准。 太阳能电池板阵列层之间采用双层EVA材料以及TPT复合材料，组件气密性好，抗潮，抗紫外线好，不容易老化。 直流接线盒：采用密封防水、高可靠性多功能ABS塑料接线盒，耐老化防水防潮性能好；连接端采用易操作的专用公母插头，使用安全、方便、可靠。 带有旁路二极管能减少局部阴影而引起的损害。 工作温度：-40℃～+90℃ 使用寿命可达20年以上，衰减小于20%。 三、问题集锦： 1、什么是太阳能电池 答：太阳能电池是基于半导体的光伏效应将太阳辐射直接转换为电能的半导体器件。现在商品化的太阳能电池主要有以下几种类型：单晶硅太阳能电池、多晶硅太阳能电池、非晶硅太阳能电池，目前还有碲华镉电池、铜铟硒电池、纳米氧化钛敏化电池、多晶硅薄膜太阳能电池及有机太阳能电池等。 晶体硅（单晶、多晶）太阳能电池需要高纯度的硅原料，一般要求纯度至少是%，也就是一千万个硅原子中最多允许2个杂质原子存在。硅材料是用二氧化硅（SiO2，
1.How to build a business that lasts100years 0:11Imagine that you are a product designer.And you've designed a product,a new type of product,called the human immune system.You're pitching this product to a skeptical,strictly no-nonsense manager.Let's call him Bob.I think we all know at least one Bob,right?How would that go? 0:34Bob,I've got this incredible idea for a completely new type of personal health product.It's called the human immune system.I can see from your face that you're having some problems with this.Don't worry.I know it's very complicated.I don't want to take you through the gory details,I just want to tell you about some of the amazing features of this product.First of all,it cleverly uses redundancy by having millions of copies of each component--leukocytes,white blood cells--before they're actually needed,to create a massive buffer against the unexpected.And it cleverly leverages diversity by having not just leukocytes but B cells,T cells,natural killer cells,antibodies.The components don't really matter.The point is that together,this diversity of different approaches can cope with more or less anything that evolution has been able to throw up.And the design is completely modular.You have the surface barrier of the human skin,you have the very rapidly reacting innate immune system and then you have the highly targeted adaptive immune system.The point is,that if one system fails,another can take over,creating a virtually foolproof system. 1:54I can see I'm losing you,Bob,but stay with me,because here is the really killer feature.The product is completely adaptive.It's able to actually develop targeted antibodies to threats that it's never even met before.It actually also does this with incredible prudence,detecting and reacting to every tiny threat,and furthermore, remembering every previous threat,in case they are ever encountered again.What I'm pitching you today is actually not a stand-alone product.The product is embedded in the larger system of the human body,and it works in complete harmony with that system,to create this unprecedented level of biological protection.So Bob,just tell me honestly,what do you think of my product? 2:47And Bob may say something like,I sincerely appreciate the effort and passion that have gone into your presentation,blah blah blah-- 2:56(Laughter) 2:58But honestly,it's total nonsense.You seem to be saying that the key selling points of your product are that it is inefficient and complex.Didn't they teach you 80-20?And furthermore,you're saying that this product is siloed.It overreacts, makes things up as it goes along and is actually designed for somebody else's benefit. I'm sorry to break it to you,but I don't think this one is a winner.
关于工作的优秀英语演讲稿 Different people have various ambitions. Some want to be engineers or doctors in the future. Some want to be scientists or businessmen. Still some wish to be teachers or lawers when they grow up in the days to come. Unlike other people, I prefer to be a farmer. However, it is not easy to be a farmer for Iwill be looked upon by others. Anyway,what I am trying to do is to make great contributions to agriculture. It is well known that farming is the basic of the country. Above all, farming is not only a challenge but also a good opportunity for the young. We can also make a big profit by growing vegetables and food in a scientific way. Besides we can apply what we have learned in school to farming. Thus our countryside will become more and more properous. I believe that any man with knowledge can do whatever they can so long as this job can meet his or her interest. All the working position can provide him with a good chance to become a talent. 1 ————来源网络整理，仅供供参考
一，太阳能光电产品计算 下面以1kW输出功率，每天使用6个小时为例，介绍一下计算数据： 1.首先应计算出每天消耗的瓦时数(包括逆变器的损耗)： 通常逆变器的转换效率为90%（国内企业研制的大功率光伏逆变器最高转换率 已达98.8%），则当输出功率为P 1=1kW时，则实际需要输出功率应为P 2 =1kW/90％ =1.11kW；若按每天使用6小时，则耗电量为W 1 =1.11kW*6小时=6.66kWh。 2.蓄电池的选择： 按照蓄电池一次充满后连续放电（非浮充状态下）可供负载一天（6小时）使用 蓄电池采用规格: 2400WH/12V。 蓄电池容量：2400WH/12V=200AH，蓄电池每日放电量 6.66kw/12v=555Ah，即每天（6小时使用时间）的用电量为12V555Ah。蓄电池的最大放电深度最好保持在70%以内, 所以输入应为：W 2 =W 1 /0.7=6.66kwh/0.7=9.51kWh。 总共容量的计算：555Ah/0.7=792.85Ah≈800Ah，实际没有800AH的容量，可以用200AH四组就可以了． 3.太阳能电池容量的计算与当地的地理位置、太阳辐射、气侯等因素有关。首先计算标准辐照度下当地的年平均日照时数H（h） H＝年辐射总量（kcal/cm2）×1.63（Wh/kcal） 365×0.1（W/cm2） 式中0.1W/cm2是25℃，AM1.5光谱时的辐照度，也是太阳能电池的标准测试条件。 表1 我国各类地区太阳能年辐射量 将年总辐射量代入公式，可得到各地区标准辐照度下当地的年平均日照时数H （h），结果如表1 按每日有效日照时间为H小时计算，再考虑到充电效率和充电过程中的损耗，充电过程中，太阳能电池板的实际使用功率为70％。 太阳能电池板的输出功率应为P 3 =9.51kWh/H/70%=13.585/H(W)。 太阳能峰值功率WP是在标准条件下：辐射强度1000W/m2，大气质量AM15，电池温度25℃条件下，太阳能电池的输出功率。太阳能电池的额定输出功率与转换效率有关，一般来讲，单位面积的电池组件，转换效率越高，其输出功率越大。太阳能电池目前的转换效率一般在14-17%之间，每平方米的太阳能电池组件输出功率约140-170WP. 面积功率*面积=功率 我们按照面积电池（m2）光电转换效率为15%计算，假设此时太阳光的总功率为 1000W/m2组件的功率为P 3 =13.585/H(kW)
个人先进事迹简介 01 在思想政治方面,xxxx同学积极向上,热爱祖国、热爱中国共产党,拥护中国共产党的领导.利用课余时间和党课机会认真学习政治理论,积极向党组织靠拢. 在学习上,xxxx同学认为只有把学习成绩确实提高才能为将来的实践打下扎实的基础,成为社会有用人才.学习努力、成绩优良. 在生活中,善于与人沟通,乐观向上,乐于助人.有健全的人格意识和良好的心理素质和从容、坦诚、乐观、快乐的生活态度,乐于帮助身边的同学,受到师生的好评. 02 xxx同学认真学习政治理论,积极上进,在校期间获得原院级三好生,和校级三好生,优秀团员称号,并获得三等奖学金. 在学习上遇到不理解的地方也常常向老师请教,还勇于向老师提出质疑.在完成自己学业的同时,能主动帮助其他同学解决学习上的难题,和其他同学共同探讨,共同进步. 在社会实践方面,xxxx同学参与了中国儿童文学精品“悦”读书系,插画绘制工作,xxxx同学在班中担任宣传委员,工作积极主动,认真负责,有较强的组织能力.能够在老师、班主任的指导下独立完成学院、班级布置的各项工作. 03 xxx同学在政治思想方面积极进取,严格要求自己.在学习方面刻苦努力,不断钻研,学习成绩优异,连续两年荣获国家励志奖学金；作
为一名学生干部,她总是充满激情的迎接并完成各项工作,荣获优秀团干部称号.在社会实践和志愿者活动中起到模范带头作用. 04 xxxx同学在思想方面,积极要求进步,为人诚实,尊敬师长.严格 要求自己.在大一期间就积极参加了党课初、高级班的学习,拥护中国共产党的领导,并积极向党组织靠拢. 在工作上,作为班中的学习委员,对待工作兢兢业业、尽职尽责 的完成班集体的各项工作任务.并在班级和系里能够起骨干带头作用.热心为同学服务,工作责任心强. 在学习上,学习目的明确、态度端正、刻苦努力,连续两学年在 班级的综合测评排名中获得第1.并荣获院级二等奖学金、三好生、优秀班干部、优秀团员等奖项. 在社会实践方面,积极参加学校和班级组织的各项政治活动,并 在志愿者活动中起到模范带头作用.积极锻炼身体.能够处理好学习与工作的关系,乐于助人,团结班中每一位同学,谦虚好学,受到师生的好评. 05 在思想方面,xxxx同学积极向上,热爱祖国、热爱中国共产党,拥护中国共产党的领导.作为一名共产党员时刻起到积极的带头作用,利用课余时间和党课机会认真学习政治理论. 在工作上,作为班中的团支部书记,xxxx同学积极策划组织各类 团活动,具有良好的组织能力. 在学习上,xxxx同学学习努力、成绩优良、并热心帮助在学习上有困难的同学,连续两年获得二等奖学金. 在生活中,善于与人沟通,乐观向上,乐于助人.有健全的人格意 识和良好的心理素质.
尊敬的领导，老师，亲爱的同学们， 大家好！我是5班的梁浩东。今天早上我坐车来学校的路上，我仔细观察了路上形形色色的人，有开着小车衣着精致的叔叔阿姨，有市场带着倦容的卖各种早点的阿姨，还有偶尔穿梭于人群中衣衫褴褛的乞丐。于是我问自己，十几年后我会成为怎样的自己，想成为社会成功人士还是碌碌无为的人呢，答案肯定是前者。那么十几年后我怎样才能如愿以偿呢，成为一个受人尊重，有价值的人呢？正如我今天演讲的题目是：自主管理。 大家都知道爱玩是我们孩子的天性，学习也是我们的责任和义务。要怎样处理好这些矛盾，提高自主管理呢？ 首先，我们要有小主人翁思想，自己做自己的主人，要认识到我们学习，生活这一切都是我们自己走自己的人生路，并不是为了报答父母，更不是为了敷衍老师。 我认为自主管理又可以理解为自我管理，在学习和生活中无处不在，比如通过老师，小组长来管理约束行为和同学们对自身行为的管理都属于自我管理。比如我们到一个旅游景点，看到一块大石头，有的同学特别兴奋，会想在上面刻上：某某某到此一游话。这时你就需要自我管理，你需要提醒自己，这样做会破坏景点，而且是一种素质低下的表现。你设想一下，如果别人家小孩去你家墙上乱涂乱画，你是何种感受。同样我们把自主管理放到学习上，在我们想偷懒，想逃避，想放弃的时候，我们可以通过自主管理来避免这些，通过他人或者自己的力量来完成。例如我会制定作息时间计划表，里面包括学习，运动，玩耍等内容的完成时间。那些学校学习尖子，他们学习好是智商高于我们吗，其实不然，在我所了解的哪些优秀的学霸传授经验里，就提到要能够自我管理，规范好学习时间的分分秒秒，只有辛勤的付出，才能取得优异成绩。 在现实生活中，无数成功人士告诉我们自主管理的重要性。十几年后我想成为一位优秀的，为国家多做贡献的人。亲爱的同学们，你们们？让我们从现在开始重视和执行自主管理，十几年后成为那个你想成为的人。 谢谢大家！
关于工作的英语演讲稿 【篇一：关于工作的英语演讲稿】 关于工作的英语演讲稿 different people have various ambitions. some want to be engineers or doctors in the future. some want to be scientists or businessmen. still some wish to be teachers or lawers when they grow up in the days to come. unlike other people, i prefer to be a farmer. however, it is not easy to be a farmer for iwill be looked upon by others. anyway,what i am trying to do is to make great contributions to agriculture. it is well known that farming is the basic of the country. above all, farming is not only a challenge but also a good opportunity for the young. we can also make a big profit by growing vegetables and food in a scientific way. besides we can apply what we have learned in school to farming. thus our countryside will become more and more properous. i believe that any man with knowledge can do whatever they can so long as this job can meet his or her interest. all the working position can provide him with a good chance to become a talent. 【篇二：关于责任感的英语演讲稿】 im grateful that ive been given this opportunity to stand here as a spokesman. facing all of you on the stage, i have the exciting feeling of participating in this speech competition. the topic today is what we cannot afford to lose. if you ask me this question, i must tell you that i think the answer is a word---- responsibility. in my elementary years, there was a little girl in the class who worked very hard, however she could never do satisfactorily in her lessons. the teacher asked me to help her, and it was obvious that she expected a lot from me. but as a young boy, i was so restless and thoughtless, i always tried to get more time to play and enjoy myself. so she was always slighted over by me. one day before the final exam, she came up to me and said, could you please explain this to me? i can not understand it. i
太阳能电池板与蓄电池配置计算公式 一：首先计算出电流： 如：12V蓄电池系统； 30W的灯2只，共60瓦。 电流=60W-12V= 5A 二：计算出蓄电池容量需求： 如：路灯每夜累计照明时间需要为满负载7小时(h); (如晚上8：00 开启，夜11：30 关闭1 路，凌晨4：30 开启2 路，凌晨5：30 关闭) 需要满足连续阴雨天5 天的照明需求。(5 天另加阴雨天前一夜的照明，计6 天) 蓄电池=5A X7h X(5 + 1)天=5A X42h= 210AH 另外为了防止蓄电池过充和过放，蓄电池一般充电到90%左右；放电余留20%左右。 所以210AH也只是应用中真正标准的70%左右。 三：计算出电池板的需求峰值(WP)： 路灯每夜累计照明时间需要为7小时(h); ★：电池板平均每天接受有效光照时间为小时(h) ； 最少放宽对电池板需求20%的预留额。 W- = (5A X7h X120%— WP-= WP=162(W)
光伏发电系统计算方法 光伏系统的规模和应用形式各异，如系统规模跨度很大，小到几瓦的太阳能庭院灯，大到MV级的太阳能光伏电站。其应用形式也多种多样，在家用、交通、通信、空间应用等诸多领域都能得到广泛的应用。尽管光伏系统规模大小不一，但其组成结构和工作原理基本相同。 太阳能发电系统由太阳能电池组、太阳能控制器、蓄电池（组）组成。如输出电源为交流220V或11 0V,还需要配置逆变器。各部分的作用为： （一）太阳能电池板：太阳能电池板是太阳能发电系统中的核心部分，也是太阳能发电系统中价值最高的部分。其作用是将太阳的辐射能力转换为电能，或送往蓄电池中存储起来，或推动负载工作。 （二）太阳能控制器：太阳能控制器的作用是控制整个系统的工作状态，并对蓄电池起到过充电保 护、过放电保护的作用。在温差较大的地方，合格的控制器还应具备温度补偿的功能。其他附加功能如光控开关、时控开关都应当是控制器的可选项； （三）蓄电池：一般为铅酸电池，小微型系统中，也可用镍氢电池、镍镉电池或锂电池。其作用是在有光照时将太阳能电池板所发出的电能储存起来，到需要的时候再释放出来。 （四）逆变器：在很多场合，都需要提供220VAC 110VAC的交流电源。由于太阳能的直接输出一般 都是12VDC 24VDC 48VDC为能向220VAC的电器提供电能，需要将太阳能发电系统所发出的直流电 能转换成交流电能，因此需要使用DC-AC逆变器。在某些场合，需要使用多种电压的负载时，也要用到DC-DC逆变器，如将24VDC的电能转换成5VDC的电能（注意，不是简单的降压）。光伏系统的设计包括两个方面：容量设计和硬件设计。