Search

Understanding and tailoring the physical behaviour of halide perovskites under practical environments is critical for designing efficient and durable optoelectronic devices. Here, we report that continuous light illumination leads to >1% contraction in the out-of-plane direction in two-dimensional hybrid perovskites, which is reversible and strongly dependent on the specific superlattice packing. X-ray photoelectron spectroscopy measurements show that constant light illumination results in the accumulation of positive charges in the terminal iodine atoms, thereby enhancing the bonding character of inter-slab I–I interactions across the organic barrier and activating out-of-plane contraction. Correlated charge transport, structural and photovoltaic measurements confirm that the onset of the light-induced contraction is synchronized to a threefold increase in carrier mobility and conductivity, which is consistent with an increase in the electronic band dispersion predicted by first-principles calculations. Flux-dependent space-charge-limited current measurement reveals that light-induced interlayer contraction activates interlayer charge transport. The enhanced charge transport boosts the photovoltaic efficiency of two-dimensional perovskite solar cells up to 18.3% by increasing the device’s fill factor and open-circuit voltage. Light-induced contraction in the out-of-plane direction in two-dimensional (2D) hybrid perovskites enables the realization of high-efficiency 2D perovskite solar cells.

International audience; Despite the remarkable optoelectronic properties of halide perovskites, achieving reproducible field effect transistor (FET) action in polycrystalline films at room temperature has been challenging and represents a fundamental bottleneck for understanding electronic charge transport in these materials. In this work, we report halide perovskite-based FET operation at room temperature with negligible hysteresis. Extensive measurements and device modeling reveal that incorporating high-k dielectrics enables modulation of the channel conductance. Furthermore, continuous bias cycling or resting allows dynamical reconfiguration of the FETs between p-type behavior and ambipolar FET with balanced electron and hole transport and an ON/OFF ratio up to 104 and negligible degradation in transport characteristics over 100 cycles. These results elucidate the path for achieving gate modulation in perovskite thin-films and provide a platform to understand the interplay between the perovskite structure, and external stimuli such as photons, fields and functional substrates, which will lead to novel and emergent properties.

Transition metal dichalcogenides (TMDCs) and two-dimensional organic and inorganic hybrid lead halide perovskites (2DPVSKs) have emerged as highly promising materials for ultralight and ultrathin optoelectronics application. They both exhibit tunability of electronic properties such as band structure, and they can form heterostructures with various types of two-dimensional materials for novel physical properties not observed in single components. However, TMDCs exhibit poor emission efficiency due to defect states and direct-to-indirect interband transition, and 2DPVSKs suffer from poor stability in ambient atmosphere. Here we report that fabrication of TMDC-on-2DPVSK heterostructures using a solvent-free process leads to novel optical transitions unique to the heterostructure which arise from the hybrid interface and exhibit a strong photoluminescence. Moreover, a two orders of magnitude enhancement of the photoluminescence as compared to WS2 emission is observed. The TMDC on top of 2DPVSK also significantly improves the stability as compared to bare 2DPVSK. Enhanced emission can be explained by electronic structure modification of TMDC by novel interfacial interactions between TMDC and 2DPVSK materials, which shows promise of the heterostructure for high efficiency and stable optoelectronic devices.

Electron sources are a critical component in a wide range of applications such as electron-beam accelerator facilities, photomultipliers, and image intensifiers for night vision. We report efficient, regenerative and low-cost electron sources based on solution-processed halide perovskites thin films when they are excited with light with energy equal to or above their bandgap. We measure a quantum efficiency up to 2.2% and a lifetime of more than 25 h. Importantly, even after degradation, the electron emission can be completely regenerated to its maximum efficiency by deposition of a monolayer of Cs. The electron emission from halide perovskites can be tuned over the visible and ultraviolet spectrum, and operates at vacuum levels with pressures at least two-orders higher than in state-of-the-art semiconductor electron sources., Electron sources play as important component in a wide range of applications. Here, the authors demonstrate efficient, regenerative, and low-cost electron sources based on solution-processed halide perovskite thin films with quantum efficiency up to 2.2% and a lifetime of more than 25 h.

Ruddlesden–Popper halide perovskites are 2D solution-processed quantum wells with a general formula A2A’n-1MnX3n+1, where optoelectronic properties can be tuned by varying the perovskite layer thickness (n-value), and have recently emerged as efficient semiconductors with technologically relevant stability. However, fundamental questions concerning the nature of optical resonances (excitons or free carriers) and the exciton reduced mass, and their scaling with quantum well thickness, which are critical for designing efficient optoelectronic devices, remain unresolved. Here, using optical spectroscopy and 60-Tesla magneto-absorption supported by modeling, we unambiguously demonstrate that the optical resonances arise from tightly bound excitons with both exciton reduced masses and binding energies decreasing, respectively, from 0.221 m0 to 0.186 m0 and from 470 meV to 125 meV with increasing thickness from n equals 1 to 5. Based on this study we propose a general scaling law to determine the binding energy of excitons in perovskite quantum wells of any layer thickness., Hybrid 2D layered perovskites are solution-processed quantum wells whose optoelectronic properties are tunable by varying the thickness of the inorganic slab. Here Blancon et al. work out a general behavior for dependence of the excitonic properties in layered 2D perovskites.

International audience; Recently, mixed-cation perovskites have promised enhanced performances concerning stability and efficiency in optoelectronic devices. Here, we report a systematic study on the effects of cation alloying on polaronic properties in cation-alloyed perovskites using first principle calculations. We find that cation alloying significantly reduces the polaron binding energies for both electrons and holes compared to pure methylammonium lead iodide (MAPbI3). This is rationalized in terms of crystal symmetry reduction that causes polarons to be more delocalized. Electron polarons undergo large Jahn-Teller distortions (~15−30%) while hole polarons tend to shrink lattice by ~5%. Such different lattice distortion footprints could be utilized to distinguish the type of polarons. Finally, our simulations show that a Cs, formamidinium (FA) and MA mixture can effectively minimize polaron binding energy while weakly affecting band gap, in a good agreement with experimental findings. These modeling results can guide future development of halide perovskite materials compositions for optoelectronic applications.

Transition metal dichalcogenides (TMDCs) and two-dimensional organic and inorganic hybrid lead halide perovskites (2DPVSKs) have emerged as highly promising materials for ultralight and ultrathin optoelectronics application. They both exhibit tunability of electronic properties such as band structure, and they can form heterostructures with various types of two-dimensional materials for novel physical properties not observed in single components. However, TMDCs exhibit poor emission efficiency due to defect states and direct-to-indirect interband transition, and 2DPVSKs suffer from poor stability in ambient atmosphere. Here we report that fabrication of TMDC-on-2DPVSK heterostructures using a solvent-free process leads to novel optical transitions unique to the heterostructure which arise from the hybrid interface and exhibit a strong photoluminescence. Moreover, a two orders of magnitude enhancement of the photoluminescence as compared to WS

International audience; Three-dimensional organic–inorganic perovskites have emerged as one of the most promising thin-film solar cell materials owing to their remarkable photophysical properties, which have led to power conversion efficiencies exceeding 20 per cent, with the prospect of further improvements towards the Shockley–Queisser limit for a single‐junction solar cell (33.5 per cent). Besides efficiency, another critical factor for photovoltaics and other optoelectronic applications is environmental stability and photostability under operating conditions. In contrast to their three-dimensional counterparts, Ruddlesden–Popper phases—layered two-dimensional perovskite films—have shown promising stability, but poor efficiency at only 4.73 per cent. This relatively poor efficiency is attributed to the inhibition of out-of-plane charge transport by the organic cations, which act like insulating spacing layers between the conducting inorganic slabs. Here we overcome this issue in layered perovskites by producing thin films of near-single-crystalline quality, in which the crystallographic planes of the inorganic perovskite component have a strongly preferential out-of-plane alignment with respect to the contacts in planar solar cells to facilitate efficient charge transport. We report a photovoltaic efficiency of 12.52 per cent with no hysteresis, and the devices exhibit greatly improved stability in comparison to their three-dimensional counterparts when subjected to light, humidity and heat stress tests. Unencapsulated two-dimensional perovskite devices retain over 60 per cent of their efficiency for over 2,250 hours under constant, standard (AM1.5G) illumination, and exhibit greater tolerance to 65 per cent relative humidity than do three-dimensional equivalents. When the devices are encapsulated, the layered devices do not show any degradation under constant AM1.5G illumination or humidity. We anticipate that these results will lead to the growth of single-crystalline, solution-processed, layered, hybrid, perovskite thin films, which are essential for high-performance opto-electronic devices with technologically relevant long-term stability.

We present photoluminescence studies of individual semiconducting single-wall carbon nanotubes at room and cryogenic temperatures. From the analysis of spatial and spectral features of nanotube photoluminescence, we identify characteristic signatures of unintentional exciton localization. Moreover, we quantify the energy scale of exciton localization potentials as ranging from a few to a few tens of millielectronvolts and stemming from both environmental disorder and shallow covalent side-wall defects. Our results establish disorder-induced crossover from the diffusive to the localized regime of nanotube excitons at cryogenic temperatures as a ubiquitous phenomenon in micelle-encapsulated and as-grown carbon nanotubes.

International audience; State-of-the-art light emitting diodes (LEDs) are made from high-purity alloys of III-V semiconductors or small molecules, but high fabrication cost and conplicated synthetic process have limited their widespread use for large area solid-state lighting applications. Here we report efficient and stable LEDs processed from solution with tunable color enabled by using phase-pure two-dimensional (2D) Ruddlesden-Popper (RP) layered perovskites with a formula (CH3(CH2)3NH3)2(CH3NH3)n-1PbnI3n+1 (n=3-5 in this report). By using controlled vertically oriented of crystal in the thin-films that facilitate efficient charge injection and transport, we obtain efficient electroluminescence with a radiance of 35 W Sr-1 cm-2 at 744 nm with an ultra-low turn-on voltage of 1V. Finally, operational stability tests suggest that phase purity is strongly correlated to stability. Phase-pure 2D perovskites exhibit >14 hours of stable operation at peak operating conditions with no droop at current-densities of several Amperes/cm2 in comparison to mixtures of 2D/3D mixture or 3D perovskites, which degrade within minutes.

International audience; Halide perovskites have demonstrated remarkable performance in optoelectronic applications. Despite extraordinary progress, questions remain about device stability. We report an in-depth computational study of small polaron formation, electronic structure, charge density, and reorganization energies of several experimentally relevant halide perovskites using isolated clusters. Local lattice symmetry, electronic structure, and electron-phonon coupling are interrelated in polaron formation in these materials. To illustrate this, first principles calculations are performed on (MA/Cs/FA)Pb(I/Br)3 and MASnI3. Across the materials studied, electron small polaron formation is manifested by Jahn-Teller like distortions in the central octahedron, with apical PbI bonds expanding significantly more than the equatorial bonds. In contrast, hole polarons cause the central octahedron to uniformly contract. This difference in manifestation of electron and hole polaron formation can be a tool to determine what is taking place in individual systems to systematically control performance. Other trends as the anion and cations are changed, are established for optimization in specific optoelectronic applications.

Large-crystal perovskite films The performance of organic-inorganic hybrid perovskite planar solar cells has steadily improved. One outstanding issue is that grain boundaries and defects in polycrystalline films degrade their output. Now, two studies report the growth of millimeter-scale single crystals. Nie et al. grew continuous, pinhole-free, thin iodochloride films with a hot-casting technique and report device efficiencies of 18%. Shi et al. used antisolvent vapor-assisted crystallization to grow millimeter-scale bromide and iodide cubic crystals with charge-carrier diffusion lengths exceeding 10 mm. Science , this issue p. 522 , p. 519

A fundamental understanding of the intrinsic optoelectronic properties of atomically thin transition-metal dichalcogenides (TMDs) is crucial for its integration into high performance semiconductor devices. Here, we investigate the transport properties of chemical vapor deposition (CVD) grown monolayer molybdenum disulfide (MoS2) under photoexcitation using correlated scanning photocurrent microscopy and photoluminescence imaging. We examined the effect of local phase transformation underneath the metal electrodes on the generation of photocurrent across the channel length with diffraction-limited spatial resolution. While maximum photocurrent generation occurs at the Schottky contacts of semiconducting (2H-phase) MoS2, after the metallic phase transformation (1T-phase), the photocurrent peak is observed toward the center of the device channel, suggesting a strong reduction of native Schottky barriers. Analysis using the bias and position dependence of the photocurrent indicates that the Schottky barrier heights are a few millielectron volts for 1T- and ∼ 200 meV for 2H-contacted devices. We also demonstrate that a reduction of native Schottky barriers in a 1T device enhances the photoresponsivity by more than 1 order of magnitude, a crucial parameter in achieving high-performance optoelectronic devices. The obtained results pave a way for the fundamental understanding of intrinsic optoelectronic properties of atomically thin TMDs where ohmic contacts are necessary for achieving high-efficiency devices with low power consumption.

Electronic interactions in low-dimensional nanomaterial heterostructures can lead to novel optical responses arising from exciton delocalization over the constituent materials. Similar phenomena have been suggested to arise between closely interacting semiconducting carbon nanotubes of identical structure. Such behavior in carbon nanotubes has potential to generate new exciton physics, impact exciton transport mechanisms in nanotube networks, and place nanotubes as one-dimensional models for such behaviors in systems of higher dimensionality. Here we use resonance Raman spectroscopy to probe intertube interactions in (6,5) chirality-enriched bundles. Raman excitation profiles for the radial breathing mode and G-mode display a previously unobserved sharp resonance feature. We show the feature is evidence for creation of intertube excitons and is identified as a Fano resonance arising from the interaction between intratube and intertube excitons. The universality of the model suggests that similar Raman excitation profile features may be observed for interlayer exciton resonances in 2D multilayered systems., Bundles of single-wall carbon nanotubes with enriched chirality can be used as model systems for exploring exciton physics in low-dimensional nanostructures. Here, the authors use resonant Raman spectroscopy to probe intertube interactions in bundles of (6,5)-enriched carbon nanotubes, and observe a Fano resonance arising from coupling between intertube and intratube excitons.

How perovskites have the edge Two-dimensional Ruddlesden-Popper perovskites form quantum wells by sandwiching inorganic-organic perovskite layers used in photovoltaic devices between organic layers. Blancon et al. show that if the perovskite layer is more than two unit cells thick, photogenerated excitons undergo an unusual but highly efficient process for creating free carriers that can be harvested in photovoltaic devices (see the Perspective by Bakr and Mohammed). Lower-energy local states at the edges of the perovskite layer facilitate dissociation into electrons and holes that are well protected from recombination. Science , this issue p. 1288 ; see also p. 1260

In-service cables such as stay cables and suspenders of cable-stayed bridges and suspension bridges, are subjected to dynamic loads (e.g., the vehicle loads and wind excitation). Performing vibration measurements and subsequently identifying the dynamic properties and establishing a dynamic model of cable vibration are essential for their dynamic analysis, condition assessment, and performance prediction. For example, based on the taut-string theory, the cable tension, as a critical indicator of cable performance and health state, can be computed using its frequency that can be identified from the measured cable vibration responses. Traditional contact-type wired or wireless sensors, such as accelerometers and strain gauge sensors, require physically attaching to the structure for vibration measurements, which could induce the mass effect. In addition, installing these sensors on structures is costly, time-consuming, and allows instrumentations at a limited number of places. On the other hand, digital video cameras have emerged as a cost effective and agile non-contact vibration measurement method, offering high-resolution, simultaneous, measurements. Recently, digital video camera measurements processed by advanced computer vision and machine learning algorithms have been successfully used for experimental and operational full-field vibration measurement and modal analysis. This study develops a video measurement and processing based technique that can autonomously and blindly extract the full-field dynamic parameters of cable vibration from the video measurements. In addition, by exploiting the taut string theory, full-order (as many modes as possible) dynamic parameters are also extracted. Therefore, a full-field, full-order dynamic (modal) model of cable vibration is established. Laboratory experiments are conducted to validate the developed approach.

International audience; Solution-processed organometallic perovskites have rapidly developed into a top candidate for the active layer of photovoltaic devices. Despite the remarkable progress associated with perovskite materials, many questions about the fundamental photophysical processes taking place in these devices remain open. High on the list of unexplained phenomena are very modest mobilities despite low charge carrier effective masses. Moreover, experiments elucidate unique degradation of photocurrent affecting stable operation of perovskite solar cells. These puzzles suggest that while ionic hybrid perovskite devices may have efficiencies on par with conventional Si and GaAs devices, they exhibit more complicated charge transport phenomena. Here we report the results from an in-depth computational study of small polaron formation, electronic structure, charge density, and reorganization energies using both periodic boundary conditions and isolated structures. Using the hybrid Density Functional Theory, we found that volumetric strain in a CsPbI3 cluster creates a polaron with binding energy of around 300 meV and 900 meV for holes and electrons, respectively. In the MAPbI3 (MA=CH3NH3) cluster, both volumetric strain and MA reorientation effects lead to larger binding energies at around 600 meV and 1300 meV for holes and electrons, respectively. Such large reorganization energies suggest appearance of small polarons in organometallic perovskite materials. The fact that both volumetric lattice strain and MA molecular rotational degrees of freedom can cooperate to create and stabilize polarons, indicates that in order to mitigate this problem, formamidinium (FA=HC(NH2)2) and cesium (Cs) based crystals and alloys, are potentially better materials for solar cell and other optoelectronic applications.

International audience; Organometallic perovskites have attracted considerable attention after promising developments in energy harvesting and other optoelectronic applications. However, further optimization will require a deeper understanding of the intrinsic photo-physics of materials with relevant structural characteristics. Here we investigate the dynamics of photogenerated charge carriers in large-area grain organometallic perovskite thin films via confocal time-resolved photoluminescence spectroscopy. It is found that the bimolecular recombination of free charges is the dominant decay mechanism at excitation densities relevant for photovoltaic applications. Bimolecular coefficients are found to be on the order of 10-9 cm3/s, comparable to typical direct-gap semiconductors, yet significantly smaller than theoretically expected. We also demonstrate that there is no degradation in carrier transport in these thin films due to electronic impurities. Suppressed electron-hole recombination and transport that is not limited by deep level defects provide a microscopic model for the superior performance of large-area grain hybrid perovskites for photovoltaic applications.

The electronic properties of single-walled carbon nanotubes can be altered by surface adsorption of electronic impurities or dopants. However, fully understanding the influence of these impurities is difficult because of the inherent complexity of the solution-based colloidal chemistry of nanotubes, and because of a lack of techniques for directly imaging dynamic processes involving these impurities. Here, we show that photoluminescence microscopy can be used to image exciton quenching in semiconducting single-walled carbon nanotubes during the early stages of chemical doping with two different species. The addition of AuCl3 leads to localized exciton-quenching sites, which are attributed to a mid-gap electronic impurity level, and the adsorbed species are also found sometimes to be mobile on the surface of the nanotubes. The addition of H2O2 leads to delocalized exciton-quenching hole states, which are responsible for long-range photoluminescence blinking, and are also mobile. Photoluminescence microscopy can be used to image exciton quenching in semiconducting single-walled carbon nanotubes during the early stages of chemical doping.

A general challenge in generating functional materials from nanoscale components is integrating them into useful composites that retain or enhance their properties of interest. Development of single walled carbon nanotube (SWNT) materials for optoelectronics and sensing has been especially challenging in that SWNT optical and electronic properties are highly sensitive to environmental interactions, which can be particularly severe in composite matrices. Percolation of SWNTs into aqueous silica gels shows promise as an important route for exploiting their properties, but retention of the aqueous and surfactant environment still impacts and limits optical response, while also limiting the range of conditions in which these materials may be applied. Here, we present for the first time an innovative approach to obtain highly fluorescent solution-free SWNT-silica aerogels, which provides access to novel photophysical properties. Strongly blue-shifted spectral features, revelation of new diameter-dependent gas-phase adsorption phenomena, and significant increase (approximately three times that at room temperature) in photoluminescence intensities at cryogenic temperatures all indicate greatly reduced SWNT-matrix interactions consistent with the SWNTs experiencing a surfactant-free environment. The results demonstrate that this solid-state nanomaterial will play an important role in further revealing the true intrinsic SWNT chemical and photophysical behaviors and represent for the first time a promising new solution- and surfactant-free material for advancing SWNT applications in sensing, photonics, and optoelectronics.

Photoluminescence quantum yields and nonradiative decay of the excitonic S(1) state in length fractionated (6,5) single-wall carbon nanotubes (SWNTs) are studied by continuous wave and time-resolved fluorescence spectroscopy. The experimental data are modeled by diffusion limited contact quenching of excitons at stationary quenching sites including tube ends. A combined analysis of the time-resolved photoluminescence decay and the length dependence of photoluminescence quantum yields (PL QYs) from SWNTs in sodium cholate suspensions allows to determine the exciton diffusion coefficient D = 10.7 ± 0.4 cm(2)s(-1) and lifetime τ(PL) for long tubes of 20 ± 1 ps. PL quantum yields Φ(PL) are found to scale with the inverse diffusion coefficient and the square of the mean quenching site distance, here l(d) = 120 ± 25 nm. The results suggest that low PL QYs of SWNTs are due to the combination of high-diffusive exciton mobility with the presence of only a few quenching sites.

We study excitation energy transfer in small aggregates of chirality enriched carbon nanotubes by transient absorption spectroscopy. Ground state photobleaching is used to monitor exciton population dynamics with sub-10 fs time resolution. Upon resonant excitation of the first exciton transition in (6,5) tubes, we find evidence for energy transfer to (7,5) tubes within our time resolution (

We study the photo-induced transmission change (ΔT/T) upon quasi-continuum laser excitation, for different modulation frequency and excitation intensity in films of (6, 5) enriched CoMoCat single wall nanotubes (SWNTs) embedded in a polymeric matrix. This technique probes long-lived excited species, with lifetime in the 1 ms–1 s domain. The observed line-shape of ΔT/T spectra that can be assigned to a superposition of ground state bleaching and absorption modulation. We speculate that photo-excitation creates charges which get trapped in the sample, acquiring ms lifetime. This causes bleaching of the ground state absorption due to state filling and possibly a tube diameter expansion, with consequent blue shift of the optical transition.

Knowledge of excited-state dynamics in carbon nanotubes is determinant for their prospective use in optoelectronic applications. It is known that primary photoexcitations are quasi-one-dimensional excitons, the electron–hole correlation length (‘exciton size’) of which corresponds to a finite volume in the phase space. This volume can be directly measured by nonlinear spectroscopy provided the time resolution is short enough for probing before population relaxation. Here, we report on the experimental determination of exciton size and mobility in (6, 5) carbon nanotubes. The samples are sodium cholate suspensions of nanotubes (produced by the CoMoCat method) obtained by density-gradient ultracentrifugation. By using sub-15 fs near-infrared pulses to measure the nascent bleach of the lowest exciton resonance, we estimate the exciton size to be 2.0±0.7 nm. Exciton–exciton annihilation in our samples is found to be rather inefficient so that many excitons can coexist on a single nanotube. An accurate determination of the size and diffusion length of excitons generated with single-walled nanotubes supports the Wannier–Mott picture of their behaviour, and improves the outlook for the use of nanotubes in optoelectronics and biosensing applications.

We have studied intersubband decay of E22 excitons in semiconducting carbon nanotubes experimentally and theoretically. Photoluminescence excitation line widths of semiconducting nanotubes with chiral indicess (n, m) can be mapped onto a connectivity grid with curves of constant (n-m) and (2n+m). Moreover, the global behavior of E22 linewidths is best characterized by a strong increase with energy irrespective of their (n-m) mod(3)= \pm 1 family affiliation. Solution of the Bethe-Salpeter equations shows that the E22 linewidths are dominated by phonon assisted coupling to higher momentum states of the E11 and E12 exciton bands. The calculations also suggest that the branching ratio for decay into exciton bands vs free carrier bands, respectively is about 10:1., Comment: 4 pages, 4 figures

Using density gradient ultracentrifugation and isopycnic fractionation we structurally sort traditionally prepared single-wall carbon nanotube suspensions, and show that fractions with photoluminescence (PL) quantum yields of over 1% can be isolated. Moreover, we find that traditionally prepared suspensions appear to be composed of a significant fraction of small nanotube aggregates which reduce the quantum yield of the ensembles by a factor of 5 with respect to the most photoluminescent fractions. In order to better understand the nature of the nonradiative decay processes that lead to lower ensemble PL quantum yields, nanotube ropes of diameter enriched tubes are made and compared with the aforementioned ensemble measurements. Changes in the optical properties are discussed in terms of energy transfer to metallic tubes, carbonaceous particles, and smaller band gap tubes.

We investigate exciton dynamics in isopycnically enriched (6,5) nanotube-DNA suspensions using femtosecond time-resolved pump−probe spectroscopy. The ground state recovery is characterized by a t-0.45 ± 0.03 power law behavior, indicative of a one-dimensional diffusion-limited reaction that is tentatively attributed to subdiffusive trapping of dark excitons. Spectral transients of bright singlet excitons within the E11 and E22 manifolds exhibit a photobleach (PB) and a photoabsorption (PA) signal of similar strength. The PA is blue-shifted with respect to the PB-signal by 7.5 meV and is attributed to a transition from the dark singlet exciton to a + state within the two exciton E11 manifold.

Shallow hole doping in small-diameter semiconducting carbon nanotubes with a valley degeneracy is predicted to result in the resonant ionization of excitons into free electron-hole pairs. This mechanism, which relies on the chirality of the electronic states, causes excitons to decay with high efficiencies where the rate scales as the square of the dopant density. Moreover, multiparticle exciton ionization can account for delocalized fluorescence quenching when a few holes per micrometer of tube length are present.

We report on a pump-probe study of CoMoCAT nanotube suspensions with narrow chirality distribution. Visible pump pulses and a white light continuum are used for resonant excitation of the strongest dipole allowed E 22 subband exciton in the semiconducting (6, 5) tube and for broadband probe of the resulting spectral transients between 1300 nm and 480 nm, respectively. Transient spectra show signatures of both photobleaching (PB) and photoabsorption (PA) with practically identical decay- but slightly different rise-times. The experiments reveal that apparent variations of decay rates at different wavelengths do not reflect dynamics of different relaxation processes but are a consequence of the superposition of PB and blue-shifted PA response.

International audience; While addition of electrolyte to sodium dodecyl sulfate suspensions of single-wall carbon nanotubes has been demonstrated to result in significant brightening of the nanotube photoluminescence (PL), the brightening mechanism has remained unresolved. Here, we probe this mechanism using time-resolved PL decay measurements. We find that PL decay times increase by a factor of 2 on addition of CsCl as the electrolyte. Such an increase directly parallels an observed near-doubling of PL intensity, indicating the brightening results primarily from changes in nonradiative decay rates associated with exciton diffusion to quenching sites. Our findings indicate that a reduced number of these sites results from electrolyte-induced reorientation of the surfactant surface structure that partially removes pockets of water from the tube surface where excitons can dissociate, and thus underscores the contribution of interfacial water in exciton recombination processes.

International audience; We present measurements of S1 exciton transport in (6,5) carbon nanotubes at room temperature in a colloidal environment. Exciton diffusion lengths associated with end quenching paired with photoluminescence lifetimes provide a direct basis for determining a median dif- fusion constant of approximately 7.5 cm2s-1. Our experimental results are compared to model diffusion constants calculated using a realistic exciton dispersion accounting for a logarithmic correction due to the exchange self-energy and a non-equilibrium distribution between bright and dark excitons. The intrinsic diffusion constant associated with acoustic phonon scattering is too large to explain the observed diffusion length, and as such, we attribute the observedtransport to disorder-limited diffusional transport associated with the dynamics of the colloidal interface. In this model an effective surface potential limits the exciton mean free path to the same size as that of the exciton wavefunction, defined by the strength of the electron-hole Coulomb interaction.

We present evidence for the generation of free carriers in aggregated single-wall carbon nanotubes by photoexcitation in the energetic range of the π→π(*) transition associated with the M saddle point of the graphene lattice. The underlying broad absorption culminating at 4.3 eV can be fit well with a Fano line shape that describes strong coupling of a saddle-point exciton to an underlying free electron-hole pair continuum. Moreover, it is demonstrated that transitions in this energetic region autoionize into the continuum by detecting features unique to the presence of free charges in the transient transmission spectra of the continuum-embedded second sub-band exciton, S(2).

The optical properties of selectively aggregated, nearly single chirality single-wall carbon nanotubes were investigated by both continuous-wave and time-resolved spectroscopies. With reduced sample heterogeneities, we have resolved aggregation-dependent reductions of the excitation energy of the S(1) exciton and enhanced electron-hole pair absorption. Photoluminescence spectra revealed a spectral splitting of S(1) and simultaneous reductions of the emission efficiencies and nonradiative decay rates. The observed strong deviations from isolated tube behavior are accounted for by enhanced screening of the intratube Coulomb interactions, intertube exciton tunneling, and diffusion-driven exciton quenching. We also provide evidence that density gradient ultracentrifugation can be used to structurally sort single-wall carbon nanotubes by aggregate size as evident by a monotonic dependence of the aforementioned optical properties on buoyant density.

We study excitation energy and charge transfer in small aggregates of chirality enriched carbon nanotubes by transient absorption spectroscopy. Ground state photobleaching is used to monitor exciton population dynamics with sub-10 fs time resolution. Upon resonant excitation of the first exciton transition in (6,5) tubes, we find evidence for energy transfer to (7,5) tubes within our time resolution (< 10 fs). After pumping at high pump energies, free charge carriers are produced via exciton scattering into the underlying continuum bands. We obtain clearly distinguished photoinduced features in the visible spectral range, that allow for real-time tracing of charge carrier dynamics in carbon nanotubes on a sub-picosecond time scale.

Complex multichromophoric systems that display ultrafast excitation transfer between their constituents are of great fundamental interest in technologies such as organic photovoltaics. In this study we focus on diameter enriched single-wall carbon nanotubes (CNTs) networks with a majority being of the (6,5) chirality. It has been recently found that excitation of the first excitonic transition triggers energy transfer (ET) between the (6,5) and the (7,5) CNTs, with ≈10 fs time constant [1]. The aim of this study is to further analyze the transfer processes occurring between the CNTs, with the surprising observation of an ultrafast charge transfer (CT) process.

Transient polarized pump/probe transmission changes upon pulse photoexcitation at 1.6 eV were measured in chiral enriched carbon nanotubes ensembles of both metallic and semiconductor characters, in a very broad probe spectral range from mid-IR to visible, combining several ultrafast laser systems. We identify a photoinduced absorption band at $\ensuremath{\sim}0.7\text{ }\text{eV}$ as due to hot Dirac fermions in the metallic tubes and measure its dynamics. The hot carriers equilibrate within the electronic system in $\ensuremath{\sim}400\text{ }\text{fs}$ and with the lattice within $\ensuremath{\sim}2\text{ }\text{ps}$; in agreement with hot Dirac fermions thermalization in graphene. We also assign excitonic transitions in the semiconducting tubes, mainly of (6,5) chirality; in particular, an intraexciton transition at $\ensuremath{\sim}0.4\text{ }\text{eV}$ represents a lower limit for the exciton binding energy.

Pump-probe spectroscopy with 10 fs time resolution is performed on (6,5) carbon nanotubes. We decompose the spectra into contributions from the first and second exciton,demonstrating their electronic correlation.

We excite and detect coherent phonons in semiconducting (6,5) carbon nanotubes via a sub-10-fs pump-probe technique. Simulation of the amplitude and phase profile via time-dependent wave packet theory yields excellent agreement with experimental results under the assumption of molecular excitonic states and allows determining the electron-phonon coupling strength for the two dominant vibrational modes.

Aqueous suspensions of single-wall carbon nanotubes (SWNTs) prepared by traditional methods from the supernatant of ultracentrifuged micellar SWNT mixtures are shown to be strongly heterogeneous with respect to the quantum yield of their constituents. The heterogeneities are isolated using preparative ultracentrifugation in iodixanol density gradients and are optically characterized by photoluminescence and linear absorption spectroscopy. We find that the most buoyant fractions with a density of 1.055 ± 0.005 g·cm-3 have the highest photoluminescence quantum yield η of 1.1%, a factor of 5 higher than that of the supernatant. Denser fractions with lower η make up for about 2/3 of these samples and contain mostly small ropes in which the quantum yield is reduced by nonradiative decay through coupling to metallic tubes.

Current state-of-the-art photovoltaics utilize high purity single-crystal semiconductors grown by sophisticated, high temperature crystal-growth processes. Alternative emerging solar cell technologies based on solution-processed materials promise to deliver power at lower cost, but so far have had limited success due to the presence of defects in the bulk and at the grain boundaries. Improving the crystal size and quality of solution-processed films could lead to unprecedented improvements in photovoltaic performance. However, large-scale crystal growth is challenging in solution-based deposition where molecular kinetics are limited by the low processing temperature. Here, for the first time we demonstrate organometallic perovskites thin-films composed of extra-ordinarily large crystalline grains (1-2 mm), fabricated using a new processing technique. The increase in the grain-size directly correlates to a dramatic increase in the efficiency, as a direct consequence of increased charge carrier mobility and reduction in defect densities. Our measured efficiency values approach ~18%, which is among the highest reported. These devices are extremely robust, exhibiting no degradation with voltage sweep direction or the rate at which the voltage was scanned. We anticipate that this technique for growing high quality large-area crystals at low temperature with low defect concentration will be universally applicable to a wide variety of other material systems thus providing a solution to a long-standing scientific challenge of overcoming polydispersity, defects and grain boundary recombination in solution-processed thin-films. From the perspective of the global photovoltaics community, these results are expected to lead the field towards the synthesis of wafer-scale crystalline perovskites necessary for the fabrication of high-efficiency single-junction and hybrid (semiconductor and perovskite) tandem planar cells. In addition, I will also discuss design principles that govern the photostability in these systems.

Selective photothermal interaction using dye enhancement has proven to be effective in minimizing surrounding tissue damage and delivering energy to target tissue. During laser irradiation, the process of photon absorption and thermal energy diffusion in the target tissue and its surrounding tissue are crucial. Such information allows the selection of proper operating parameters such as dye concentrations, laser power, and exposure time for optimal therapeutic effect. Combining the Monte Carlo method for energy absorption and the finite difference method for heat diffusion, the temperature distributions in target tissue and surrounding tissue in dye enhanced laser photothermal interaction are obtained. Different tissue configurations and dye enhancement are used in the simulation, and different incident beam sizes are also used to determine optimum beam sizes for various tissue configurations. Our results show that the algorithm developed in this study could predict the thermal outcome of laser irradiation. Our simulation indicates that with appropriate absorption enhancement of the target tissue, the temperature in the target tissue and in the surrounding tissue can be effectively controlled. This method can be used for optimization of lesion treatment using laser photothermal interactions. It may also provide guidance for laser immunotherapy in cancer treatment, since the immunological responses are believed to be related to tissue temperature changes.

We present a pump-probe study of exciton dynamics in semiconducting (6,5) single-wall carbon nanotubes (SWNTs). Optical transients from individual tubes and small bundles (n

The precise and selective destruction of tumor tissue through hyperthermia, when combined with immunoadjuvant applications, has shown promise in the treatment of cancer. The combination of an 805-nm laser and indocyanine green (ICG) has been shown to be highly effective in laboratory experiments in satisfying both the precision and selectivity requirements of such a procedure. A systematic study was conducted to further understand the interaction of the laser and the dye, using both Monte Carlo simulation and direct temperature measurement in gel phantom and chicken tissue. A phantom system, with a spherical, dye-enhanced target in the center, was constructed to simulate a tumor buried in deep tissue. The temperature increase in the target tissue under the irradiation of the 805-nm laser was significantly higher than the surrounding tissue. The same tumor-tissue configuration was also used in the theoretical simulation. The theoretical and experimental results for the temperature distributions and the treatment parameters can be used to provide the optimal thermal effect in cancer treatment.

When studying the optical properties of single-walled carbon nanotubes (SWCNTs), it is known that the dispersing agent(s) play an important role in the observed optical and electronic properties as well as in the separation of tubes. However, fundamental and applied reports are hindered by poor control over the interaction between the dispersant molecules and the nanotubes. Here I will discuss how interfacial chemistry affects the separation of SWCNTs and their optical properties. Compelling spectroscopic evidence has shown that surfactant composition, concentration, and structure are crucial factors to separate SWCNTs and obtain highly stable and fluorescent nanotube dispersions.

The search for environmentally clean energy sources has spawned a wave of research into the use of carbon nanomaterials for photovoltaic applications. In particular, research using semiconducting single-walled carbon nanotubes has undergone dramatic transformations due to the availability of high quality samples through colloidal separation techniques. This has led to breakthrough discoveries on how energy and charge transport occurs in these materials and points to applications in energy harvesting. We present a review of the relevant photophysics of carbon nanotubes that dictate processes important for integration as active and passive material elements in thin film photovoltaics. Fundamental processes ranging from light absorption and internal conversion to exciton transport and dissociation are discussed in detail from both a spectroscopic and a device perspective. We also give a perspective on the future of these fascinating materials to be used as active and passive material elements in photovoltaics.

not Available.

not Available.

not Available.