半导体中光子检测方案(激光产品)

智能文字提取功能测试中

RSC Advances View Article OnlineRSC AdvancesPaper Check for updates Cite this： RSC Adv.， 2019， 9， 12153 Received 19th February 2019Accepted 11th April 2019 DOI： 10.1039/c9ra01273g rsc.li/rsc-advances Introduction View Article OnlineView Journal|View Issue NIR-to-visible upconversion in quantum dots viaa ligand induced charge transfer statet Noga Meir，iD a lddo Pinkas )band Dan Oron *a Nanomaterials that possess the ability to upconvert two low-energy photons into a single high-energyphoton are of great potential to be useful in a variety of applications. Recent attempts to realizeupconversion (UC) in semiconducting quantum dot (QD) systems focused mainly on fabrication ofheterostructured colloidal double QDs， or by using colloidal QDs as sensitizers for triplet-tripletannihilation in organic molecules. Here we propose a simplified approach， in which colloidal QDs arecoupled to organic thiol ligands and UC is achieved via a charge-transfer state at the molecule-dotinterface. We synthesized core/shell CdSe/CdS QDs and replaced their native ligands with thiophenolmolecules. The alignment of the molecular HOMO with respect to the QD conduction band resulted inthe formation of a new charge-transfer transition from which UC can be promoted. We performeda series of pump-probe experiments and proved the non-linear emission exhibited by these QDs is theresult of UC by sequential photon absorption， and further characterized the QD-ligand energylandscape by transient absorption. Finally， we demonstrate that this scheme can also be applied in a QDsolid. Photon upconversion (UC) is a non-linear process in which twoor more low-energy photons are converted to one high-energyphoton. In the context of nanomaterials， realization of UCholds great promise for the use of these nanoparticles ina variety of applications. One example for that is the potentialto overcome the Shockley-Queisser limit in photovoltaicdevices，2 by upconverting low-energy photons in the near-infrared (NIR) range of the solar spectrum， whose energiesare below the band gap of the absorber material.Upconvertingnanoparticles can also be beneficial for bioimaging， as tags fortwo-photon excitation fluorescence， which enables the use ofNIR radiation for excitation， thus allowing deeper penetrationto scattering biological samples and providing a higher signal-to-noise ratio and reduced autofluorescence. In contrast toconventional two-photon dyes， UC potentially has an addi-tional advantage since it requires a lower photon fluence，consequently reducing the damage caused to the specimen by ( "Department of Physics of Complex Systems， Weizmann Insti t ute of Science， Rehovot7610001， Israel. E-mail： dan.oron@weizmann.ac.il ) ( Department of Chemical Research Support， Weizmann Insti t ute of Science， Reh o vot 7610001， Israel ) ( tElectronic supplementary information (ESI) available： A c a lculation for the electron and hole energy l e vels， HRTEM image of the CdSe/CdS QDs， a fit of t h eband-edge l ifetime for the QD-TP system， the upconverted spectrum o f t heQD-TP， e x amples of the i n strument re s ponse tr a nsient， p u mp-probe con t rolexperiment， additional TA r esults and a more d etailed explanation regarding the trends observed in the pump-probe results. See DOI： 10.1039/c9ra01273g ) the excitation beam. UC nanoparticles have also recently beensuggested for controlled optogenetic stimulation deep insidescattering tissue.4，5 Unlike other non-linear phenomena such as two-photonabsorption or sum-frequency generation (SFG)， UC is anincoherent process whose mechanism is based on thesequential absorption of photons by long-lived， metastablestates. As such， the requirements for efficient UC includealadder-like arrangement of the energy states，a mechanismwhich inhibits cooling of hot charge carriers and relativelylong-lived excited states (from which UC can be promoted).UC by sequential photon absorption in nanocrystals cantake place via several mechanisms， such as excited-stateabsorption (ESA)， energy-transfer UC (ETU) and photonavalanche (PA). In all three mechanisms the absorptionleads to population of highly excited states， however theydiffer by the number of fluorophores involved in theprocess.，8 Another mechanism， more typical to the use oforganic molecules， is triplet-triplet annihilation (TTA)，’ inwhich the energy from an excited triplet state of a sensitizermolecule is transferred to an excited triplet state of anemitter (also known as activator) molecule， followed byinteraction between two emitter molecules，resulting in oneemitter in the ground state and another in the excitedsinglet state (from which UC emission occurs). UC systemsbased on this mechanism can exhibit relatively high UCefficiencies， however it has several drawbacks such as pho-tobleaching and limited availability of NIR fluorophoresdue to the use of organic molecules. Although the majority of attempts to realize UC in nano-materials were based on the use of host materials doped withrare-earth metals or transition metal ions，8-10 in recent yearsa different approach was implemented， employing semi-conducting colloidal D heterostructures (CQDs).1.Theadvantage of using CQDs for UC lies in their higher colortunability and absorption cross section. UC is facilitated byrecent developments in the field of QDs synthesis， allowingthe fabrication of a great variety of QDs in different compo-sitions and architectures. The ability to achieve UC in suchsystems depends mostly on rational design of the energylandscape in the quantum dot heterostructure， allowingcouplingof two quantum emitters， as well as confinement ofat least one of the charge carriers (electron or hole) indifferent areas of the crystal. This can be accomplished bycareful synthesis of the QD heterostructure in specific archi-tectures， such that the two quantum emitters are spatiallyseparated by a potential barrier. This was previously demon-strated for UC both in the vis-to-vis11，12 and the NIR-to-vis13，14regime. In both of these realizations， a low band-gap dot isfirst excited， followed by further excitation of the hole eitherby intraband absorption or by an Auger mechanism. Theexcited hole can relax into the higher band gap dot， whichthen recombines with the spatially delocalized electron. Otherrecently introduced methods for achieving UC by the use ofcolloidal QDs are to couple them to organic moleculesexhibiting TTA， where the QDs function as sensitizers for UCby TTA，15-17 or relying on hot carrier injection from a plas-monic nanoparticle.18 Yet， the range of accessible emissionenergies in the QD-TTA scheme is still limited by the energygap (So →Ta) of the emitter molecule， which typically doesnot go below ~1.1 eV. Alternatively， NIR-to-vis UC was ach-ieved by cooperative sensitization of organic molecules by PbSQDs， leading to UC efficiency which significantly exceedsprevious reports.Notably， the latter requires couplingbetween neighboring QDs and was thus only realized in a QDfilm architecture. In this work， we suggest a different approach and demon-strate how UC can be realized in a coupled QD-ligand system， inwhich the upconverted luminescence is emitted from the QD.This particular system is composed of a II-VI core/shell QD andthiol ligands attached to the surface of the crystal. The attach-ment of these specific ligands gives rise to a new electronictransition， occurring at the interface between the molecule andthe QD surface. Thus， the first transition in this UC schemeinvolves excitation of a charge-transfer state between the HOMOof the ligand and the conduction band of the QD. Froma synthetic standpoint， this ligand-to-dot UC system is mucheasier to fabricate than previous dot-to-dot UC design， since itrequires fewer synthetic steps and has an overall highersynthetic yield. Moreover， relying on a charge-transfer transi-tion， this poses no requirements on the HOMO-LUMO gap ofthe organic molecule. We use a pump-probe experimentalsetup to discern the nature of the non-linear emission exhibitedby these QDs and prove it is the result of UC by sequentialphoton absorption. Finally， we observed this type of upconver-sion both in the case of isolated QDs dispersed in an organic Experimental solvent， and in a close-packed array of the QDs， which wasdeposited on a solid substrate. Synthesis Materials. Cadmium oxide (CdO， 99.99%)， oleic acid (90%)，tri-n-octylphosphine(TOP， 90%)，，trioctylphosphine oxide(TOPO， 99%)， octadecene (ODE， 90%)， selenium (Se， 99.999%)，sulfur (S， 99.5%)，1，4-benzenedithiol(1，4-BDT， 99%)， diethyleneglycol (DEG， 99%)， oleic acid (OA， 90%) and all organic solventswere purchased from Sigma-Aldrich and used without furtherpurification. Octadecylphosphonic acid (ODPA) was purchasedfrom PCI Synthesis. Thiophenol (TP， 98%) was purchased fromMerck. CdSe cores synthesis.2 60 mg Cdo， 280 mg ODPA and 3 gTOPO were degassed in a 50 ml flask under vacuum at 120°C for30 min. The flask was then heated to 380 °C under Ar flow. At~300°C， 1.8 ml TOP was injected to the flask. At 380 °C，a solution of 58 mg Se in 0.5 ml TOP was quickly injected intothe flask and the heating was removed immediately. The flaskwas cooled to room temperature and the QDs were washed twicewith acetone and methanol and re-dispersed in toluene. CdS shell growth. Based on a previously published proce-dure， with some modifications.21 0.5 M Cd-oleate in ODE wasprepared by degassing 1.28 g Cdo， 10 ml OA and 10 ml ODE at100 °C for 20 min， followed by heating under Ar flow to 260 °Cuntil a color-less solution was formed. The flask was thencooled to 120 °C and degassed for 1 h before cooling to RT. Forthe Cds growth， 10 ml ODE and 200 nmol of purified CdSe dotsin toluene were degassed in a 50 ml flask under vacuum at120°C for 30 min. The flask was then heated to 180°C under Arflow. At 180°C， a mixture of 0.5 M Cd oleate in ODE and 0.5 M Sin TOP solutions were slowly injected into the flask usinga syringe pump， at a rate of 1 ml h-1. During the injection， thetemperature was gradually raised to 300 °C. The reaction wasstopped after 2 h. Ligand exchange to TP CdSe/CdS core/shell dots were purified by centrifugation withacetone and methanol and re-dispersed in toluene. 2 ml of 1 pMof purified QDs solution was mixed with 20 pl of 0.1 MTP： toluene solution and stirred for 10 min. QD film self-assembly and ligand exchange to 1，4-BDT QD film preparation was performed in a Teflon well， based ona known procedure with some modifications and additions.22-24A solution of purified QDs in toluene was gently dropped on thesurface of diethylene glycol， and the solvent was allowed toevaporate for ~2 h. Then， the film was separated from the DEGsurface by slow injection of acetonitrile. The film was collectedby removal of the DEG using a syringe pump and transferred toan ITO-coated glass substrate that was submerged in the DEG ata 30° angle. The substrate was kept under vacuum overnight toremove residual acetonitrile. For ligand-exchanged film prepa-ration， 0.03 M solution of 1，4-BDT in acetonitrile was used instead of clean acetonitrile.25 An additional step of ligandexchange was performed after 1 day， in which the films weredipped in 0.1 M solution of 1，4-BDT in acetonitrile for 10 minand then washed with clean acetonitrile. EM and optical characterization TEM images were taken at 120 kV， using a Philips CM-120transmission electron microscope. HRTEM images were takenat 200 kV， using a Themis-Z ultra high-resolution， doubleaberration-corrected transmission electron microscope. SEMimages were taken at 2 kV by an in-lens SE detector， usinga Zeiss Ultra 55 scanning electron microscope. UV-vis absorp-tion spectra were measured using a UV-vis/NIR spectropho-tometer (V-670， JASCO). Absolute O.D. was measured using anabsolute quantum yield characterization system (HamamatsuQuantaurus QY). Time-resolved fluorescence spectroscopy and pump-probeexperiment Excitation pulses were generated by a frequency tripledNd：YAGQ-switched laser oscillator pumping an optical parametricoscillator (OPO) (NT342/C/3/UVE， EKSPLA) with pulse durationsof ~5 ns at a repetition rate of 10 Hz. Excitation pulses at 480-680 nm were obtained from the OPO and focused into a 10 ×10 mm rectangular quartz cuvette (Starna Cells)， containinga solution of a low concentration of QDs dispersed in toluene，and stirred with a magnetic stirrer. For the pump-probeexperiment， the 1064 nm probe pulse was transferred througha delay line and combined with the pump pulse by a dichroicmirror. Fluorescence signals were collected at a right-angle bya 0.4 NA objective. After spectral filtering by a dielectric filter，the signal was further passed through a monochromator(SpectraPro 2150i， Acton) for complete filtering of the laserpulses from the fluorescence. Time-resolved signals weremeasured by a photo-multiplier tube (R10699， HamamatsuPhotonics， Hamamatsu City， Japan). Data was collected by a 600MHz oscilloscope (WaveSurfer 62Xs， Teledyne LeCroy). Pulseenergies were measured by a pyroelectric sensor (PE9-C， OphirOptronics). Transient absorption Transient absorption (TA) of the core/shell QDs before and afterLE was measured in toluene. The excitation (pump) pulse wasobtained from the amplified 120 fs Ti ：sapphire system (SpitfireACE) using a computer controlled OPA (TOPAS) at either550 nm or 670 nm， operating at 1 kHz and chopped at 500 Hz.The pump was followed at different delay times by a continuumwhite-light pulse， generated by focusing a part of the 800 nmfundamental beam onto a 6 mm thick CaF2 plate， creatinga supercontinuum. The CaF2 plate was translated continuouslyin a circular motion to avoid thermal damage， which wouldaffect the CaF2 white-light spectrum. For each time delay， thesignal was accumulated over 5000 pulses for optimal signal-to-noise ratio (SNR). For our QD-ligand system we used CdSe/Cds core/shell QDsand thiophenol (TP) ligands， connected to the surface of theCds shell through the S atom (Fig. 1a). CdSe QDs with a 2.8 nmdiameter were synthesized by hot-injection method and latercoated with CdS shell by slow continuous injection of Cd and Sprecursors at an elevated temperature. The final averagediameter of the QDs was 7.4 nm. Following the synthesis， theoriginal ligands attached to the QDs surface (mainly oleic acid)were replaced with TP. Further description of the QDs synthesisand the ligand exchange (LE) process can be found in theexperimental section. Due to the location of the TP HOMO levelrelative to the Cds band gap， the exchange of the QDs nativeligands with TP resulted in the formation of a trap state locatedabove the valence band of the Cds shell. Upon the formation ofan exciton in the charge-transfer transition between the Fig.1. (a) Schematic illustration of the CdSe/CdS core/shell structureof the QDs and the TPligands connected to the surface of the particle.(b) TEM image of the as-synthesized core/shell QDs. Scale bar is10 nm.(c) Schematic illustration of the QD-ligand band alignment andthe process of upconversion by sequential photon absorption. Theelectron is excited to the CdS band edge and relaxes such that it ismostly localized in the CdSe core (blue)， but is also slightly extendedinto the CdS shell (purple). After the absorption of a second photon，the hole trapped in the TP HOMO is further excited and relaxes to theCdSe， where it recombines with the electron and emits an upcon-verted photon. The TP HOMO and LUMO levels are labeled with shortdashed lines. The TP LUMO is well above all other energy levels(HOMO-LUMO gap is ~4.4 eV (ref. 26)) and is not involved in the UCprocess.The TP HOMO level is~0.4 eV above the VB of the CdSe. Thelong dashed line represents the calculated 1S(e) level of the CdSe/CdSQD (see Fig.S1t). molecule HOMO and the Cds， the electron tends to be localizedin the QD core (see Fig. S1t)， while the hole is trapped in theligand HOMO. This trapped hole can be further excited byadditional absorption of a low-energy photon，which can lead toits recombination with the electron in the CdSe core， andemission of an upconverted photon. A schematic description ofthe UC process in this system is shown in Fig. 1c. We first examine the effect of the LE on the linear opticalproperties of the QDs. As can be seen in Fig. 2a， the absorptionspectrum of the QDs before and after the LE is mostly unaltered.Close examination of the lowest-energy absorption band byabsolute O.D. measurements reveals a very small red-shift ofabout 3 nm following the ligand exchange (inset of Fig. 2a)，while the spectral shape of the absorption band is preserved.This can be attributed to slight exciton delocalization caused bythe thiophenol ligand shell.27，28 No additional features were Wavelength (nm) Fig.22(a) Absorption spectra of the core/shell QDs， before and afterthe ligand exchange to thiophenol. The inset depicts the opticaldensity around the first exciton absorption peak， showing a ~3 nmred-shift following the ligand exchange. (b) Emission spectra of thecore/shell QDs， before and after ligand exchange. The emissionspectrum of the post-LE dots (red line) was scaled up by a factor of 10for clarity. The inset shows time-resolved fluorescence signal of theQD taken at 570 nm (PL peak)， before and after LE. observed in the absorption spectrum of the ligand-exchangeddots compared to the as-synthesized ones. Fig. 2b shows the photoluminescence spectrum of the QDsbefore and after ligand exchange (blue and red lines， respec-tively) and time-dependent PL traces measured at the peak ofthe emission (Fig. 2b inset). The PL spectrum and lifetimemeasurements were performed by exciting a dilute sample ofthe QDs with a 5 ns pulsed laser at 480 nm， filtering the time-resolved fluorescence signal through a monochromator andcollecting it with a photomultiplier tube. Following the attach-ment of TP ligands， the band-edge emission of the QDs isconsiderably quenched to ~3% of its original value and is red-shifted by ~10 nm. This finding supports the claim regardingthe location of the TP HOMO level， causing the TP molecule tobe an efficient hole trap when attached to this type of QDs，consistent with the work of Meijerink and coworkers.29 Addi-tionally， we notice the lifetime measured at the peak of the PL(570 nm)， which is comprised of both the radiative and non-radiative decay routes of the excited state， is significantlyshortened， indicating as well quenching of the radiativerecombination pathway in the QD band gap. The measured lifetime is fit to a biexponential decay Aie()+Age(a)+c，convolved with the instrument response function (~8 ns， seeFig. S3t). In both as-synthesized and ligand-exchanged dots thePL trace exhibited a very fast component (well below 5 ns) whichcorresponds to a fast decay to non-radiative trap states， anda slower component dominated by radiative recombination inthe CdSe core. This longer lifetime component decreased from30 ns to 16 ns as a result of the ligand exchange， and theamplitude ratio between the non-radiative and radiative life-time was decreased by a factor of 20， in agreement with thedecrease of the PL intensity. We then turned to examine the non-linear photophysicalproperties of our QD-TP system. Excitation with a 680 nmbeam， which is well below the energy of the QD first excitonabsorption band leads to a non-linear photoluminescencesignal whose spectral shape is identical to that of the linear PLspectrum of the QDs (see Fig. S4 in the ESIt). However， thiscould also be the result of simultaneous two-photon absorptionin the CdSe core， and indeed we observed a similar PL spectralshape for the original QDs prior to ligand exchange. Thus， inorder to gain more compelling evidence for the occurrence ofUC， and to differentiate between this process and two-photonabsorption， we performed a series of PL pump-probe experi-ments. The setup for the pump-probe measurements is sche-matically presented in Fig. 3a. After the initial excitation of theQDs solution by a5 ns pulse ranging from 650 nm to 680 nm，the sample is excited again by a second pulse at a longerwavelength of 1064 nm which is delayed by approximately 15 ns.This delay was chosen in order to prevent overlap of the twopulses. A typical pump-probe experiment performed on the QD-TPsystem， in which the QDs are excited using a 670 nm beam ispresented in Fig. 3b. The photoluminescence transient from theQD sample， collected at the peak of the PL spectrum (580 nm)，was measured three times： once only with the pump pulse， a) Time (ns) T ime (ns) d) 1or e) Fig.3s (a) Schematic illustration of the optical setup used for the pump-probe measurements. (b) Typical result of a pump-probe measurementof the QD-TP system， excited by a 670 nm pump with a pulse energy of 100 uJ and a 1064 nm probe at 600 pJ， delayed by 15 ns. The PL tracesdepicted are detected at 580 nm， and correspond to excitation of the QDs with either the pump (purple line)， the probe (green line)， or the pumpand delayed probe (yellow line). The ratio between the fluorescence signal of the combined excitation (pump + probe) and the sum of theindividual excitations (labeled with a blue dashed line) for this measurement was ~3. (c) Typical result of a pump-probe measurement performedon the same sample presented in (b)， with a 32 ns delay. The sample was excited with a 100 pJ pump. The probe intensity was adjusted such thatthe energy density at the sample will be the same for both delays. (d) Dependence of the upconverted PL on the probe pulse energy， for fixedpump intensities. The data was fit to a linear saturation assuming Poissonian distribution for the excitation probability (black dashed lines). (e)Dependence of the upconverted PL on the pump pulse energy， for fixed probe intensities. Black dashed lines depict linear fits of the data. (f)Dependence of the Fl ratio with the probe pulse energy， for fixed pump excitation intensity， summarizing all the pump-probe measurementsperformed on this sample with 670 nm pump and 1064 nm probe at increasing intensities. All the results presented in (d)-(f) relate to themeasurements taken with a 15 ns delay. another only with the probe pulse and a third time with both thepump and probe pulses. As can be seen in the figure， both of theindividual excitations - either the red pump or IR probe- canlead to upconverted PL (purple and green time traces， corre-spondingly)， likely by two-photon absorption (certainly for theIR probe) or by consecutive absorption of two photons withsimilar energies. However， when the QDs are excited by both thepump and probe， we notice a significant increase in the PLsignal that corresponds to the arrival of the probe pulse. Thisbehavior was consistent for all the measurements performed onthe QD-TP system， and was not observed in any of the pre-LEQDs. As an additional control， we tested for UC PL in QDswhich underwent ligand exchange to octanethiol， an aliphaticthiol， but observed none (see Fig. S6 in the ESIt). When inte-grating over 30 ns from the arrival of the probe pulse，we findthat the fluorescence in the case of the dual excitation is aboutthree times higher than the combination of the two individualexcitations (for the measurement depicted in Fig. 3b). There-fore， it is clear that a considerable contribution to the non-linear PL in this system comes from UC by sequential absorp-tion of two photons where the intermediate state lifetime is atleast several tens of nanoseconds. This is further corroboratedby increasing the delay time between the pump and probe to 32 nns (Fig. 3c). The intensity of the upconverted PL decreased onaverage by ~35%， indicating a significant portion of the QDspopulation remained in the excited state. As a side note， it isimportant to clarify the appearance of what seems to look likea second emission peak resulting from the 670 nm excitation.This is an artifact caused by double-humped temporal shape ofthe pulse generated by the OPO close to its degeneracy point.Therefore， the two peaks correspond to the same electronictransition in the QD-TP system. This artifact becomes detect-able due to the shortening of the emission lifetime upon ligandexchange， which becomes comparable with the instrumentresponse time. We repeated the pump-probe measurement for a series ofincreasing pump and probe excitation intensities. Fig. 3d and eplots the upconverted luminescence extracted from themeasurements taken with a 15 ns delay against either pump orprobe intensity， after subtraction of the background emissionresulting from two-photon absorption of either the pump orprobe. As expected， the upconverted PL scales linearly with boththe pump and the probe energy for low excitation intensities.Further increasing the probe intensity results in linear satura-tion of the upconverted PL (Fig. 3d) at approximately 1000-1500可(corresponds to ~1 J cm). Similarly to an above-band gap excitation in QDs， this can be modeled using Poissonian distributioonn (P(n)=)， in win hwihcihch tthhee probability to exciten excitons (or in our case， generate n trials to transfer an excitedhole by intraband absorption) is given by the distribution param-eter =，when Isat is the saturation energy. Thus， the proba-Isatbility to transfer the excited hole above the potential barrier willdepend on P(n=1)=1--PP((00))==11-- eexxpp(-()). Addition-ally， since the upconverted PL as a function of the pump intensitydoes not reach saturation in the region of our measurement， andbased on the maximal pump energy density (which corresponds to~0.18 J cm-2)， we can give an upper bound for the 670 nmabsorption cross section of ~6×10-17 cm’， almost two orders ofmagnitude lower than the characteristic cross section of the bandedge transition in these QDs. Fig. 3f plots the ratio of the total luminescence intensity inthe 15 ns pump-probe experiment to the sum of the lumines-cence upon excitation with the pump pulse alone or the probepulse alone. The result of a series of such pump-probe experi-ments using different excitation intensities for the pump beamat 670 nm is presented. Since the upconverted PL signalcontains contributions from both simultaneous and sequentialabsorptions of photon pairs from the pump and probe beam， itwill be proportional to the following expression： When P is the intensity of the pump beam， P is the intensity ofthe probe beam and a， b are coefficients attributed to theprobability of absorption of two photons from the individual(pump or probe， respectively) excitation pulses. From thisexpression it is easy to see that for weak probe intensities， theexpected increase of the Fl ratio will be steeper when lowerexcitation intensitieess araer eus eudsed slopera)Additionallyhemaximal Fl ratio will be obtained under the condition ofaP12=bP22. This expectation is also corroborated by ourexperimental findings， as can be seen in Fig. 3e， where themaximal Fl ratio appears at higher probe intensities with the increase of the pump intensity. Further details regarding thisanalysis are included in the ESI.t Overall， the results presentedin Fig.3d-fcorroborate our claim regarding the feasibility of UCby sequential absorption of two photons in the QD-TP system. Although the results of the pump-probe experiments pre-sented so far in this manuscript provide proof for the existenceof UC in our QD-TP system， it does not give clear indication forthe spectral position of the meta-stable state from which UCtakes place. Time-resolved PL transients of both types of QDs(before and after the LE) did not clearly reveal any new radiativetransitions created due to the attachment of the new ligandsAdditionally， no new absorption features appeared in the QDsspectrum following the LE process (Fig.2a) that can be attrib-uted to a direct absorption of the pump wavelength. This can beexplained by the weak oscillator strength of the charge-transfertransition， which is below the detection limit of our instru-mentation， yet still requires a closer inspection. Thus， analternative spectroscopic method is needed in order to gaininsight regarding the electronic transitions in the two types ofQD systems. We therefore performed two sets of transientabsorption (TA) measurements using a 120 fs pump pulse. Inthe first set we used a 670 nm pump(same as in our PL pump-probe experiment) at increasing excitation intensities， andexamined the bleach evolution of the first excitonic transition inthe CdSe core. The second set of measurements presented hereused a 550 nm pump. This allowed us to probe the opticallyexcited state of the system close to the band-edge and with psresolution. We first plot the difference of the particles' absorption withand without the 670 nm pump (AO.D.) against the wavelengthof the white-light probe (Fig. 4a and b)， focusing on the regionof the two lowest-energy transitions in the CdSe core， locatedaround 500 and 560 nm. This spectrum was constructed bysumming all the TA spectra taken with time delays between 25to 50 ps from the arrival of the pump pulse. In both cases(before and after LE) excitation with the 670 nm pump results ina bleach of these two transitions due to the excitation of anelectron in the CdSe core. However， a more careful examinationof the bleach properties reveals a significantly different depen-dence of the bleach intensity on the excitation power for the twodifferent QDs samples. Fig. 4c presents the dependence of a) O 3.5 Fig.4 Power-dependent transient absorption measurements with a 670 nm pump. TA spectra of the QDs， (a) before and (b) after the LE， takenbetween 25 and 50 ps after the arrival of the pump pulse， with increasing pump power. (c) Bleach intensity of the 1st excitonic transition (~560nm) in the QDs before and after LE， plotted against the 670 nm pump power. The black dashed lines depict the power law fit (y =ax) of eitherone of the QDs samples， giving a power dependence of b=1.9 for the pre-LE QDs and b=1.3 for the QDs coated with TP. bleach magnitude of the 1st exciton (at ~560 nm) with thepump power， before and after LE. Both curves showed a good fitto a power law y =ax， where the as-synthesized QDs showeda nearly quadratic power dependence (b=1.9)， typical of two-photon absorption， while the ligand-exchanged QDs powerdependence was close to a linear one (b= 1.3)， indicatinga significant component of linear absorption. These findingssupport our claim regarding the origin of the non-linear PLexhibited by the CdSe core in both types of QD systems. As theenergy of the pump is lower than the first absorption band ofthe CdSe core， state filling of the CdSe in the pre-LE QDs willonly be allowed by absorption of photon pairs. However， in theQDs coated with TP it can be achieved either by absorption ofphoton pairs， or by absorption of single photons via the charge-transfer state， leading to a much lower exponent， and provingthat single 670 nm photons can be absorbed by the QD-TPcomplex. Due to the weak absorption induced by 670 nm excitation wewere not able to detect new electronic transitions in the ligand-exchanged QDs compared to the original ones. However， usinga 550 nm pump， which matches the band edge absorption of theCdSe core， proved to be more beneficial in this context. Since theTA measurements already clearly showed the presence of linearabsorption of the charge transfer state， we expect to observea bleach feature at a wavelength corresponding to this transition.Notably，this is difficult， since the white light continuum we use isgenerated with an 800 nm pump， limiting the accessible spectralrange. Considering this， the most distinguishable differencebetween the TA spectrum of the QDs before and after the LE(Fig. 5a) is a new bleach band found around 875 nm. As can beseen in the figure， following the LE to TP we detect a bleach in theabsorption， which did not exist prior to the LE process. Thisindicates a new transition in the QD-TP system induced by the LEprocess， whose absorption decreases upon excitation of a bandedge exciton in the QD. As such， it most likely corresponds to thecharge transfer absorption occurring at the interface between theQD and the molecules (schematically drawn in Fig. 1c). Exam-ining the kinetics probed around 875 nm for the QDs before andafter the LE (Fig. S7 in the ESIt)， we notice both types of QD systems show induced absorption， followed by a quick relaxationupon carrier cooling into a surface trap state. Fitting this relaxa-tion trace to a biexponential decay dynamics， we find the timeconstant for trapping becomes shorter due to the LE. Thisdynamic witnessed in both QD systems is typical of hole trap-ping， although in this case the time constants for relaxation arelonger than for an intra-dot trap， most likely due to slower relax-ation dynamies to the trap state， affected by the molecule-QDinterface. Following the trapping， in the case of the QDs before theLE， we witness excited state absorption (ESA) which does notrecover within the timeframe of this measurement (~2ns).However， for the QD-TP system we observe a clear bleach whichpartially recovers after ~500 ps but does not reach the inducedabsorption value observed prior to LE， even on the time scale of 2ns (Fig. 5b and c， correspondingly). This bleach indicates theexistence of a transient filling of an electronic transition arisingfrom the connection of the TP ligands to the QD surface. There-fore， the observed feature at 875 nm is likely implicated with theUC process. Notably， this is in agreement with the results ofMeijerink and coworkers on the position of aromatic thiol traps. So far we discussed the dynamics of UC in isolated QDs，suspended in an organic medium. These results show that UCin the QD-TP system is an intrinsic property； in the following weshow that this type of ligand-to-dot UC can be also realized ina QD solid. Demonstrating the feasibility of UC in dense，closely-packed QD structures is an important step towardsutilizing these non-linear processes in light harvesting appli-cations. QD films were formed by self-assembly on an immis-cible liquid surface in a Teflon well， using a previouslypublisheddprocedure， with some adaptations.22-24Briefly，a purified QD solution in toluene was gently drop-casted on thesurface of diethylene glycol (DEG)， and the toluene was allowedto evaporate completely. Then the formed QD film was sepa-rated from the DEG by slow injection of acetonitrile andtransferred1 tto anITO-coatedgglass substrate whichhwassubmerged in the DEG， by slow removal of the DEG witha syringe pump. This method of QD film preparation results inclose-packed structure of the QDs (see SEM image， Fig.6a)，which extends for tens and even hundreds of micrometers and a)4 O4 Fig. 65UC in QD solids. (a) SEM image of self-assembled QD film. (b)PL traces of a typical pump-probe experiment， performed on a QDfilm after LE to 1，4-BDT， using a 680 nm pump at 120 pJ and a 1064 nmprobe at 1200 uJ. The Fl ratio in this case was 2.8. is composed of up to 10 monolayers. For ligand-exchanged QDfilms， we used 1，4-benzenedithiol (1，4-BDT) molecules， thusachieving maximal similarity to our QD-TP system. The LEprocess was performed after the formation of the film andbefore the transfer to the substrate. Performing LE in thismanner helps maintaining the close-packed arrangement of theQDs and prevents cracks and damage to the film.5 A moredetailed description of the QD films formation and LE is givenin the experimental section. In a similar manner to the solution suspended QDs， the QDfilms also exhibited non-linear PL when subjected to excitationwith lower energy than the first electronic transition， and havealso shown a decrease in the measured lifetime of the linear PLupon LE to 1，4-BDT. Here too we performed a series of pump-probe experiments， using the same optical setup describedearlier， and a typical pump-probe measurement is presented inFig2..6b. Generally， the results were consistent with themeasurements performed on the solution-suspended QD- anincrease of the PL signal was detected with the arrival of theprobe pulse， and the overall PL in this time frame was higherthan the sum of the emission caused by 2-photon absorption ofeither the 680 or 1064 nm excitation， for all of the ligand-exchangedQD films measured. Due to the similar effect of theLE on both the solution-suspended QDs and the QD films， andthe consistent results of the pump-probe experiments， we cansafely assume that ligand-to-dot UC takes place by the samemechanism in the QD films as well as in the QD-TP solution. In conclusion， we demonstrated how UC can be realized ina coupled QD-ligand system via a mechanism which was notpreviously considered. The attachment of specific thiol ligandsto the surface of a core/shell CdSe/CdS QDs resulted in theformation of a new charge transfer transition， absorbing in theNIR spectral region. Upon excitation with energy below theband gap of the CdSe core， the exciton formed in the QD-ligandcomplex can be further promoted by intraband absorption of anadditional photon and recombine in the CdSe core， thus emit-ting an upconverted photon. We performed a series of pump-probe experiments， in which the probe pulse could onlygenerate intraband absorption. These measurements， andparticularly the linear scaling of the upconverted emission withboth pump and probe intensities， provided proof for theoccurrence of UC by sequential photon absorption. Addition-ally， transient absorption measurements helped us builda more complete picture of the UC process by providing furtherinsight regarding the spectral position of the meta-stable statefrom which UC was promoted， whose energetic position wasfound to be consistent with past studies on aromatic thiols.Finally， we showed that this system can also be constructed asa QD solid and exhibit the same optical properties as thesolution-suspended QDs， demonstrating how this UC schemecan transpire both in single QDs and in an ensemble of coupledQDs. Due to the dramatic quenching of the band edge lumines-cence by the TP ligands，the efficiency of the UC process in thisparticular coupled QD-ligand system is very low， makingquantitative assessment of the yield very difficult. Indeed， it iswell known that thiol-based ligands can dramatically quenchphotoluminescence from QDs. Nevertheless， and unlike theinherently low absorption cross section of the charge-transfertransition， this quenching is not due to an intrinsic flaw ofthis upconversion scheme. We believe that with proper choiceof the ligand， enabling better control of the energy gap betweenthe molecular HOMO and the QD conduction band， the effi-ciency of the ligand-mediated upconversion process can besignificantly enhanced， although the saturation intensity wouldlikely remain high due to the weak absorption.Moreover，and incontrast with alternative hybrid organic-inorganic upconver-sion nanoparticles based on triplet-triplet annihilation15-17 orcooperative sensitization， the spectral range of the absorptioninligand-mediated upconversion is decoupled from theHOMO-LUMO gap of the ligand molecule and the band gap ofthe QD， readily allowing for upconversion even deeper into theinfrared日spectral range. Finally， thisis scheme： presentsa dramatic simplification over upconversion schemes based ondouble quantum dots.11，13，14 All in all， and particularly whenconsidering the simplicity of the synthetic procedure， we believethese results provide an alternative route for the formation ofbroadband upconversion nanocrystals. Conflicts of interest Acknowledgements The authors thank L. Houben for performing HRTEM of theQDs， T. Udayabhaskararao for his help with the fabrication ofthe QD solids and A. Teitelboim for her assistance with the dataanalysis and for helpful discussions throughout this project.Financial support by the Israeli Science Foundation (grant 270/16) is gratefully acknowledged. References ( 1 W. Shockley and H. J. Qu e isser，J. Appl. Phys.， 1961， 32， 510 . ) ( 2 J. de Wild， A. Meijerink， J. K. Rath， W. G . J . H. M. v an Sark and R. E. I. Schropp， Energy Environ. Sci.， 2011， 4， 4835. ) ( 3 J. Zhou， Z. Liu and F. Li， Chem. Soc. Rev.，2012， 41， 1323-1349. ) ( 4 S. Hososhima， H . Yuasa， T . I s hizuka， M. R . Hoque，T. Yamashita， A. Yamanaka， E. S ugano， H. Tomita and H. Yawo， Sci. Rep.，2015，5，1-10. ) ( 5 B. Zheng， H . Wang， H. Pan， C. Liang， W. Ji， L. Zhao， H. Chen，X. Gong， X. Wu and J. C hang， ACS Nano， 2017， 1 1 ， 11898- 11907. ) ( .6 T. Trupke，M. A. Green and P. Wirfel， J. Appl. Phys.，2002，92， 4117-4122. ) ( 7 M. Haase and H.Schafer， Angew. Chem.， Int . Ed . Engl.， 2011，50，5808-5829. ) ( 8 G. Chen， H . Q iu， P. N. Prasad and X. Chen， Chem. Rev.，2014， 114，5161-5214. ) ( 9 S . Wu， G. Han， D. J.Milliron， S. Aloni， V. Altoe， D. V Talapin and B. E . Cohen， Proc. N atl. Acad. Sci. U. S . A.，2009，106，1-5. ) 10 F. Auzel， Chem. Rev.， 2004， 104，139-173. ( 11 Z. Deutsch， L. Neeman and D. Oron， Nat. Nanotechnol.，20 1 3， 8，649-653. ) ( 12 C. C. Milleville， E . Y . Chen， K. R. Lennon， J. M. Cleveland，A. Kumar，J. Z h ang， J. A . Bork， A. Tessier， J . M. L e beau， D. B. Chase， M . O . Joshua and M. F. Doty， ACS Nano， 2019， 13，489-497. ) ( 13 A. Teitelboim and D . Oron， ACS Nano， 2016， 10， 446-452. ) ( 14 N. S. Makarov， Q. Lin， J . M. P i etryga， I. R obel andV.I. Klimov， ACS Nano， 2016， 10，10829-10841. ) ( 15 M. W u， D . N. Congreve， M. W. B. Wi l son， J. J e an， N. Geva， M. Welborn， T. van Voorhis， V. Bulovic， M. G. Bawendi and M. A . Baldo， Nat. Photonics， 2015，10，31-34. ) 16 Z. Huang， X. Li， M. Mahboub， K. M. Hanson， V. M. Nichols，H. Le， M. L. Tang and C. J. Bardeen， Nano Lett.， 2015， 15，5552-5557. 17 M.Mahboub，Z. Huang and M. L. Tang， Nano Lett.， 2016，16，7169-7175. ( 18 G. V. Naik， A. J. Welch， J. A . Briggs， M . L. Solomon andJ. A. Dionne， Nano Lett.， 2017，17，4583-4587. ) ( 19J. L auth. G. Grimaldi. ， S. K inge， A. J. .上 Houtepen，L. D. A. Siebbeles a nd M. Scheele， Angew. Ch e m.， Int. Ed . ， 2017，56，14061-14065. ) ( 20 L. Carbone， C. Nobile， M. De Giorgi， F. Della Sala， G. Morello，P. Pompa， M. Hy t ch， E. Snoeck， A. Fiore， I. R . Franchini，M. N adasan， A. A . F. . Silvestre， L. Chiodo， S. K Kudera， R. Cingolani， R. Krahne and L. M a nna， Nano Lett.， 20 0 7， 7， 2942-2950. ) ( 21 S. Christodoulou， G . Vaccaro，V. Pinchetti， F . D e Donato， J . Q. Grim， a . Casu， a. Genovese， G . Vicidomini， a. Diaspro，S. Brovelli ， L. Manna and I . Moreels， J. Mater. Chem. C ， 2014，2，3439. ) ( 22 A . Dong， J. Chen， P. M. Vora， J. M. Kikkawa and C. B. Murray，Nature， 2010，466，474-477. ) ( 23 M. P P. B oneschanscher， W . H . E v ers， J . J . Geuchies， T . Altantzis， B. Goris ， F. T. Rabouw， S. A. P . van Rossum，H. S . J. van der Zant， L . D. a S iebbeles， G. V an T e ndeloo，I. Swart， J. H ilhorst， A.V. P etukhov， S.Bals and D. Vanmaekelbergh， Science， 2014， 344，1377-1380. ) ( 24 A . D ong， J. Chen， S. J. Oh， W. K. Koh， F. Xiu， X. Ye， D. K. Ko， K. L. Wang， C. R. Kagan and C. B. Murray， Nano Lett.， 2011， 11，841-846. ) ( 25 A. Dong，Y. Jiao and D. J . Milliron， ACS Nano，2013，7，10978- 10984. ) ( 26 R . H. Abu-Eittah and R. H. Hi l al， Appl. Spectrosc.， 1972， 2 6 ， 270. ) ( 27 K. O. Aruda， V. A . Amin， C . M. Thompson， B. .I Lau， A. B. Nepomnyashchii and E. A. Weiss， Langmuir， 2 0 16，32 ， 3354-3364. ) 28 M. S. Azzaro， A. Dodin， D. Y. Zhang， A. P. Willard andS. T. Roberts， Nano Lett.，2018， 18， 3259-3270. ( 29 S. F. Wuister， C. de Mello Donega a n d A. M e ijerink，J. P h ysChem. B，2004，108，17393-17397. ) ( 30 A. Avidan， I. Pinkas and D. Oron， ACS Nano， 2012， 6，3063- 3069. ) RSC Adv.， ， his journal is @ The Royal Society of Chemistry Nanomaterials that possess the ability to upconvert two low-energy photons into a single high-energy photon are of great potential to be useful in a variety of applications. Recent attempts to realize upconversion (UC) in semiconducting quantum dot (QD) systems focused mainly on fabrication of heterostructured colloidal double QDs, or by using colloidal QDs as sensitizers for triplet–triplet annihilation in organic molecules. Here we propose a simplified approach, in which colloidal QDs are coupled to organic thiol ligands and UC is achieved via a charge-transfer state at the molecule–dot interface. We synthesized core/shell CdSe/CdS QDs and replaced their native ligands with thiophenol molecules. The alignment of the molecular HOMO with respect to the QD conduction band resulted in the formation of a new charge-transfer transition from which UC can be promoted. We performed a series of pump–probe experiments and proved the non-linear emission exhibited by these QDs is theresult of UC by sequential photon absorption, and further characterized the QD–ligand energylandscape by transient absorption. Finally, we demonstrate that this scheme can also be applied in a QD solid.