Mechanistic Understanding and Rational Design of Quantum Dot/Mediator Interfaces for Efficient Photon Upconversion.

The semiconductor-nanocrystal-sensitized, three-component upconversion system has made great strides over the past 5 years. The three components (i.e., triplet photosensitizer, mediator, and emitter) each play critical roles in determining the input and output photon energy and overall quantum efficiency (QE). The nanocrystal photosensitizer converts the absorbed photon into singlet excitons and then triplet excitons via intersystem crossing. The mediator accepts the triplet exciton via either direct Dexter-type triplet energy transfer (TET) or sequential charge transfer (CT) while extending the exciton lifetime. Through a second triplet energy-transfer step from the mediator to the emitter, the latter is populated in its lowest excited triplet state. Triplet-triplet annihilation (TTA) between two triplet emitters generates the emitter in its bright singlet state, which then emits the upconverted photon. Quantum dots (QD) have a tunable band gap, large extinction coefficient, and small singlet-triplet energy losses compared to metal-ligand charge-transfer complexes. This high triplet exciton yield makes QDs good candidates for photosensitizers. In terms of driving triplet energy transfer, the triplet energy of the mediator should be slightly lower than the triplet exciton energy of the QD sensitizer for a downhill energy landscape with minimal energy loss. The same energy cascade is also required for the transfer from the mediator to the emitter. Finally, the triplet energy of the emitter must be slightly larger than one-half of its singlet energy to ensure that TTA is exothermic. Optimization of the sensitizer, mediator, and emitter will lead to an increase in the anti-Stokes shift and the total quantum efficiency. Evaluating each individual step's efficiency and kinetics is necessary for the understanding of the limiting factors in existing systems.This review summarizes chalcogenide QD-based photon upconversion systems with a focus on the mechanistic aspects of triplet energy transfer conducted by the Tang and Lian groups. Via time-resolved spectroscopy, the rates and major loss pathways associated with the two triplet energy-transfer steps were identified. The studies are focused on the near-infrared (NIR) to visible (VIS) PbS-tetracene-based systems as they allow systematic control of the QD, mediator, and emitter. Our results show that the mediator triplet state is mostly formed by direct TET from the QD and the transfer rate is influenced by the density of bound mediator molecules. Charge transfer, a loss pathway, does not produce triplet excitons and can be minimized by adding an inert shell to the QD. This transfer rate decreases exponentially with the distance between the QD and mediator molecule. The second TET rate was found to be much slower than the diffusion-limited collision rate, which results in the triplet lifetime of the mediator being the main factor limiting its efficiency. Finally, the total quantum efficiency can be calculated using these measured quantities including the TET1 and TET2 efficiencies. The agreement between calculated and measured quantum efficiencies suggests a firm understanding of QD-sensitized photon upconversion. We believe the above conclusions are general and should be widely applicable to similar systems, including singlet fission in hybrid organic-nanocrystal materials.

Low temperature radical initiated hydrosilylation of silicon quantum dots.

The photophysics of silicon quantum dots (QDs) is not well understood despite their potential for many optoelectronic applications. One of the barriers to the study and widespread adoption of Si QDs is the difficulty in functionalizing their surface, to make, for example, a solution-processable electronically-active colloid. While thermal hydrosilylation of Si QDs is widely used, the high temperature typically needed may trigger undesirable side-effects, like uncontrolled polymerization of the terminal alkene. In this contribution, we show that this high-temperature method for installing aromatic and aliphatic ligands on non-thermal plasma-synthesized Si QDs can be replaced with a low-temperature, radical-initiated hydrosilylation method. Materials prepared via this low-temperature route perform similarly to those created via high-temperature thermal hydrosilylation when used in triplet fusion photon upconversion systems, suggesting the utility of low-temperature, radical-initiated methods for creating Si QDs with a range of functional behavior.

Enhanced Near-Infrared-to-Visible Upconversion by Synthetic Control of PbS Nanocrystal Triplet Photosensitizers.

Photon upconversion employing semiconductor nanocrystals (NCs) makes use of their large and tunable absorption to harvest light in the near-infrared (NIR) wavelengths as well as their small gap between singlet and triplet excited states to reduce energy losses. Here, we report the highest QY (11.8%) thus far for the conversion of NIR to yellow photons by improving the quality of the PbS NC. This high QY was achieved by using highly purified lead and thiourea precursors. This QY is 2.6 times higher than from NCs prepared with commercially available lead and sulfide precursors. Transient absorption spectroscopy reveals two reasons for the enhanced QY: longer intrinsic exciton lifetimes of PbS NCs and the ability to support a longer triplet lifetime for the surface-bound transmitter molecule. Overall, this results in a higher efficiency of triplet exciton transfer from the PbS NC light absorber to the emitter and thus a higher photon upconversion QY.

Primary amines enhance triplet energy transfer from both the band edge and trap state from CdSe nanocrystals.

In this study, the role that primary amines play during triplet energy transfer from photoexcited CdSe nanocrystals (NCs) was examined. Colloidally synthesized CdSe NCs were placed in varying concentrations of 1-propyl- or 1-octylamine, with and without 2-anthracenecarboxylic acid transmitter ligands attached. This primary amine increases upconversion quantum yield approximately 5-fold. Further addition of amine does not improve photon upconversion, as CdSe NC photoluminescence (PL) increases at the expense of triplet energy transfer. Transient absorption measurements show that the amine plays three key roles. Primary amines enhance NC PL by decreasing the nonradiative decay rate, increase the rate of triplet energy transfer, and enable the broad trap state in these CdSe NCs to participate in triplet photosensitization.

Designing Transmitter Ligands That Mediate Energy Transfer between Semiconductor Nanocrystals and Molecules.

Molecular control of energy transfer is an attractive proposition because it allows chemists to synthetically tweak various kinetic and thermodynamic factors. In this Perspective, we examine energy transfer between semiconductor nanocrystals (NCs) and π-conjugated molecules, focusing on the transmitter ligand at the organic-inorganic interface. Efficient transfer of triplet excitons across this interface allows photons to be directed for effective use of the entire solar spectrum. For example, a photon upconversion system composed of semiconductor NCs as sensitizers, bound organic ligands as transmitters, and molecular annihilators has the advantage of large, tunable absorption cross sections across the visible and near-infrared wavelengths. This may allow the near-infrared photons to be harnessed for photovoltaics and photocatalysis. Here we summarize the progress in this recently reported hybrid upconversion platform and point out the challenges. Since triplet energy transfer (TET) from NC donors to molecular transmitters is one of the bottlenecks, emphasis is on the design of transmitters in terms of molecular energetics, photophysics, binding affinity, stability, and energy offsets with respect to the NC donor. Increasing the efficiency of TET in this hybrid platform will increase both the up- and down-conversion quantum yields, potentially exceeding the Shockley-Queisser limit for photovoltaics and photocatalysis.

Hybrid Molecule-Nanocrystal Photon Upconversion Across the Visible and Near-Infrared.

The ability to upconvert two low energy photons into one high energy photon has potential applications in solar energy, biological imaging, and data storage. In this Letter, CdSe and PbSe semiconductor nanocrystals are combined with molecular emitters (diphenylanthracene and rubrene) to upconvert photons in both the visible and the near-infrared spectral regions. Absorption of low energy photons by the nanocrystals is followed by energy transfer to the molecular triplet states, which then undergo triplet-triplet annihilation to create high energy singlet states that emit upconverted light. By using conjugated organic ligands on the CdSe nanocrystals to form an energy cascade, the upconversion process could be enhanced by up to 3 orders of magnitude. The use of different combinations of nanocrystals and emitters shows that this platform has great flexibility in the choice of both excitation and emission wavelengths.

Observation of multiple, identical binding sites in the exchange of carboxylic acid ligands with CdS nanocrystals.

We study ligand exchange between the carboxylic acid group and 5.0 nm oleic-acid capped CdS nanocrystals (NCs) using fluorescence resonance energy transfer (FRET). This is the first measurement of the initial binding events between cadmium chalcogenide NCs and carboxylic acid groups. The binding behavior can be described as an interaction between a ligand with single binding group and a substrate with multiple, identical binding sites. Assuming Poissonian binding statistics, our model fits both steady-state and time-resolved photoluminescence (SSPL and TRPL, respectively) data well. A modified Langmuir isotherm reveals that a CdS nanoparticle has an average of 3.0 new carboxylic acid ligands and binding constant, Ka, of 3.4 × 10(5) M(-1).

Synthetic control of isolated, single functional groups on silica surfaces.

We report control of the density of isolated, single functional groups in homogeneously mixed trichloroalkyl silanes on various silica surfaces. The functional groups are covalently bound to a silane derived from the Rink resin. This Rink-silane is reactive to any nucleophile. Control over the density of functional groups is achieved by diluting the immersion solution containing the Rink-silane with an inert silane, octadecyltrichlorsilane. The isolated nature of the functional groups is confirmed by the stochastic blinking of fluorescent single boron-dipyrromethane dyes imaged in total internal reflection geometry. The robust character of silane monolayers allows facile covalent binding and cleavage of molecular species from silica surfaces as well as general synthetic transformations to be conducted. This is shown by the covalent attachment and then cleavage of a naphthalene chromophore. This low-cost and scalable platform has great potential for use in sensing, molecular electronics, semiconductor processing, and the investigation of fundamental processes in catalysis and the kinetics of molecular association.

Nanoantenna-enhanced gas sensing in a single tailored nanofocus.

Metallic nanostructures possess plasmonic resonances that spatially confine light on the nanometre scale. In the ultimate limit of a single nanostructure, the electromagnetic field can be strongly concentrated in a volume of only a few hundred nm(3) or less. This optical nanofocus is ideal for plasmonic sensing. Any object that is brought into this single spot will influence the optical nanostructure resonance with its dielectric properties. Here, we demonstrate antenna-enhanced hydrogen sensing at the single-particle level. We place a single palladium nanoparticle near the tip region of a gold nanoantenna and detect the changing optical properties of the system on hydrogen exposure by dark-field microscopy. Our method avoids any inhomogeneous broadening and statistical effects that would occur in sensors based on nanoparticle ensembles. Our concept paves the road towards the observation of single catalytic processes in nanoreactors and biosensing on the single-molecule level.

Observations of shape-dependent hydrogen uptake trajectories from single nanocrystals.

In this work, H(2) absorption and desorption in faceted, crystalline Au/Pd core/shell nanocrystals and their interaction with a SiO(x)/Si support were studied at the single-particle level. Dark-field microscopy was used to monitor the changing optical properties of these Au/Pd nanoparticles (NPs) upon exposure to H(2) as reversible H(2) uptake from the Pd shell proceeded. Analysis of the heterogeneous ensemble of NPs revealed the H(2) uptake trajectory of each nanocrystal to be shape-dependent. Differences in particle uptake trajectories were observed for individual particles with different shapes, faceting, and Pd shell thickness. In addition to palladium hydride formation, the single-particle trajectories were able to decipher specific instances where palladium silicide formation and Au/Pd interdiffusion occurred and helped us determine that this was more frequently seen in those particles within an ensemble having thicker Pd shells. This noninvasive, plasmonic-based direct sensing technique shows the importance of single-particle experiments in catalytically active systems and provides a foundation for studying more complex catalytic processes in inhomogeneous NP systems.

CdSe nanocrystal sensitized photon upconverting film.

Here, films using CdSe nanocrystal (NC) triplet photosensitizers in conjunction with diphenylanthracene (DPA) emitters were assembled to address several challenges to practical applications for solution-based photon upconversion. By using poly(9-vinylcarbazole) as a phosphorescent host in this film, volatile organic solvents are eliminated, the spontaneous crystallization of the emitter is significantly retarded, and ∼1.5% photon upconversion quantum yield (out of a maximum of 50%) is obtained. Transient absorption spectroscopy on nanosecond-to-microsecond time scales reveals this efficiency is enabled by an exceptionally long triplet lifetime of 3.4 ± 0.3 ms. Ultimately, we find the upconversion efficiency is limited by incomplete triplet-triplet annihilation, which occurs with a rate 3-4 orders of magnitude slower than in solution-phase upconversion systems.

Bidirectional triplet exciton transfer between silicon nanocrystals and perylene.

Hybrid materials comprised of inorganic quantum dots functionalized with small-molecule organic chromophores have emerged as promising materials for reshaping light's energy content. Quantum dots in these structures can serve as light harvesting antennas that absorb photons and pass their energy to molecules bound to their surface in the form of spin-triplet excitons. Energy passed in this manner can fuel upconversion schemes that use triplet fusion to convert infrared light into visible emission. Likewise, triplet excitons passed in the opposite direction, from molecules to quantum dots, can enable solar cells that use singlet fission to circumvent the Shockley-Queisser limit. Silicon QDs represent a key target for these hybrid materials due to silicon's biocompatibility and preeminence within the solar energy market. However, while triplet transfer from silicon QDs to molecules has been observed, no reports to date have shown evidence of energy moving in the reverse direction. Here, we address this gap by creating silicon QDs functionalized with perylene chromophores that exhibit bidirectional triplet exciton transfer. Using transient absorption, we find triplet transfer from silicon to perylene takes place over 4.2 µs while energy transfer in the reverse direction occurs two orders of magnitude faster, on a 22 ns timescale. To demonstrate this system's utility, we use it to create a photon upconversion system that generates blue emission at 475 nm using photons with wavelengths as long as 730 nm. Our work shows formation of covalent linkages between silicon and organic molecules can provide sufficient electronic coupling to allow efficient bidirectional triplet exchange, enabling new technologies for photon conversion.

Tuning the Quantum Dot (QD)/Mediator Interface for Optimal Efficiency of QD-Sensitized Near-Infrared-to-Visible Photon Upconversion Systems.

Lead sulfide (PbS) quantum dots (QDs) have shown promising performance as a sensitizer in infrared-to-visible photon upconversion systems. To investigate the key design rules, we compare three PbS-sensitized upconversion systems using three mediator molecules with the same tetracene triplet acceptor at different distances from the QD. Using transient absorption spectroscopy, we directly measure the triplet energy-transfer rates and efficiencies from the QD to the mediator and from the mediator to the emitter. With increasing distance between the mediator and PbS QD, the efficiency of the first triplet energy transfer from the QD to the mediator decreases because of a decrease in the rate of this triplet energy-transfer step, while the efficiency of the second triplet energy transfer from the mediator to the emitter increases because of a reduction in the QD-induced mediator triplet state decay. The latter effect is a result of the slow rate constant of the second triplet energy-transfer process, which is 3 orders of magnitude slower than the diffusion-limited value. The combined results lead to a net decrease of the steady-state upconversion quantum yield with distance, which could be predicted by our kinetic model. Our result shows that the QD/mediator interface affects both the first and second triplet energy transfer processes in the photon upconversion system, and the QD/mediator distance has an opposite effect on the efficiencies of the first and second triplet energy transfer. These findings provide important insight for the further rational improvement of the overall efficiency of QD-based upconversion systems.

Correlating carrier type with frontier molecular orbital energy levels in organic thin film transistors of functionalized acene derivatives.

We investigate the relationship between the charge carrier type in organic thin film transistors (OTFTs) and molecular energy levels. We examine a series of functionalized acenes that collectively have their HOMOs range from -4.9 eV to -5.6 eV and LUMOs range from -2.8 eV to -3.7 eV, as measured by cyclic voltammetry. Placed together, these 20 molecules allow us to chart the transition from OTFTs that display only hole transport, to ambipolar, to solely electron transport. Specifically, we note that for octadecyltrimethoxysilane (OTS) treated substrates, with top contact gold electrodes, electron injection and transport occurs when the LUMO < -3.15 eV, while hole injection and transport ceases when the HOMO < -5.6 eV. Ambipolar transport prevails when molecules have HOMO/ LUMO levels within the aforementioned range. This is seen across channel lengths ranging from 50-150 microm and using only gold as electrodes. This empirical plot is the first time such a detailed study has been made on the onset of charge injection and transport for a class of organic semiconductors. It provides guidelines for future molecular design.

Pentaceno[2,3-b]thiophene, a hexacene analogue for organic thin film transistors.

Hexacene and larger fused rings remain elusive targets for chemists. Here, we report a hexacene-like molecule containing six linearly fused rings, specifically a pentacene molecule fused with a terminal thiophene ring, pentaceno[2,3-b]thiophene. It can be purified and isolated as a purple-black powder at ambient conditions. This molecule has a low HOMO-LUMO gap of 1.75 eV in o-DCB and an optical band gap of 1.58 eV in thin film. Top contact organic thin film transistors (OTFTs) were made, and atomic force microscopy (AFM) reveals a dendritic thin film growth characteristic of pentacene. An OTFT mobility of 0.574 cm(2)/V s was measured for pentaceno[2,3-b]thiophene under nitrogen.

Ambipolar, high performance, acene-based organic thin film transistors.

We present a high performance, ambipolar organic field-effect transistor composed of a single material. Ambipolar molecules are rare, and they can enable low-power complementary-like circuits. This low band gap, asymmetric linear acene contains electron-withdrawing fluorine atoms, which lower the molecular orbital energies, allowing the injection of electrons. While hole and electron mobilities of up to 0.071 and 0.37 cm2/V.s, respectively, are reported on devices measured in nitrogen, hole mobilities of up to 0.12 cm2/V.s were found in ambient, with electron transport quenched. These devices were fabricated on octadecyltrimethoxysilane-treated surfaces at a substrate temperature of 60 degrees C.

Thin film structure of tetraceno[2,3-b]thiophene characterized by grazing incidence X-ray scattering and near-edge X-ray absorption fine structure analysis.

Understanding the structure-property relationship for organic semiconductors is crucial in rational molecular design and organic thin film process control. Charge carrier transport in organic field-effect transistors predominantly occurs in a few semiconductor layers close to the interface in contact with the dielectric layer, and the transport properties depend sensitively on the precise molecular packing. Therefore, a better understanding of the impact of molecular packing and thin film morphology in the first few monolayers above the dielectric layer on charge transport is needed to improve the transistor performance. In this Article, we show that the detailed molecular packing in thin organic semiconductor films can be solved through a combination of grazing incidence X-ray diffraction (GIXD), near-edge X-ray absorption spectra fine structure (NEXAFS) spectroscopy, energy minimization packing calculations, and structure refinement of the diffraction data. We solve the thin film structure for 2 and 20 nm thick films of tetraceno[2,3-b]thiophene and detect only a single phase for these thicknesses. The GIXD yields accurate unit cell dimensions, while the precise molecular arrangement in the unit cell was found from the energy minimization and structure refinement; the NEXAFS yields a consistent molecular tilt. For the 20 nm film, the unit cell is triclinic with a = 5.96 A, b = 7.71 A, c = 15.16 A, alpha = 97.30 degrees, beta = 95.63 degrees, gamma = 90 degrees; there are two molecules per unit cell with herringbone packing (49-59 degree angle) and tilted about 7 degrees from the substrate normal. The thin film structure is significantly different from the bulk single-crystal structure, indicating the importance of characterizing thin film to correlate with thin film device performance. The results are compared to the corresponding data for the chemically similar and widely used pentacene. Possible effects of the observed thin film structure and morphology on charge carrier mobility are discussed.

Ligand enhanced upconversion of near-infrared photons with nanocrystal light absorbers.

We designed and synthesized a tetracene derivative 4-(tetracen-5-yl)benzoic acid (CPT) as a transmitter ligand used in PbS/PbSe nanocrystal (NC) sensitized upconversion of near infrared (NIR) photons. Under optimal conditions, comparing CPT functionalized NCs with unfunctionalized NCs as sensitizers, the upconversion quantum yield (QY) was enhanced 81 times for 2.9 nm PbS NCs from 0.021% to 1.7%, and 11 times for 2.5 nm PbSe NCs from 0.20% to 2.1%. The surface density of CPT controls the solubility of functionalized NCs and the upconversion QY. By increasing the concentration of CPT in the ligand exchange solution, the number of CPT ligand per NC increases. The upconversion QY is maximized at a transmitter density of 1.2 nm-2 for 2.9 nm PbS, and 0.32 nm-2 for 2.5 nm PbSe. Additional transmitter ligands inhibit photon upconversion due to triplet-triplet annihilation (TTA) between two neighboring CPT molecules on the NC surface. 2.1% is the highest reported QY for TTA-based photon upconversion in the NIR with the use of earth-abundant materials.

Ligand Binding to Distinct Sites on Nanocrystals Affecting Energy and Charge Transfer.

Hybrid optoelectronic devices are attractive because they offer the promise of low-cost, roll-to-roll fabrication. Despite this, energy transfer between organic and inorganic interfaces is not well understood. Device engineering on this class of solution-processed materials generally focuses on replacing the long insulating ligands with short ones. Here, we show that energy and charge transfer between an inorganic nanocrystal (NC) donor and organic molecular acceptor is acutely sensitive to the chemical moiety linking the two species. Our results reveal that the CdS NCs have distinct binding sites for different chemical species because only resonance energy transfer (RET) is observed for the carboxylic-acid-functionalized ligand, while both RET and charge transfer are observed for the amine-functionalized ligand. We observe that the equilibrium constant for this static quenching term increases with decreasing particle size. This finding offers a new approach in the design of hybrid thin films for devices and NC probes based on RET used for imaging, sensing, signal transduction, and photon management.