We combine monolayer TMD with chiral lead halide perovskites to achieve enhanced valley polarization via chirality induced spin selective charge transfer and to demonstrate the utility of chiral perovskites as spin filters, preferentially injecting charges with one spin, depending on the handedness of the perovskite material. This elegant, yet simple way to control valley polarization does not require circularly polarized light, and can be realized at room temperature, without a need for cryogenic temperatures, usually implying expensive, bulky, complex setups. Achieving robust valley polarization in TMDs at room temperature is of great importance for practical applications of TMD based valleytronics devices in quantum information science and technology.
The two degenerate valleys, K and K’, in 2D transition metal dichalcogenides (TMD) can be used as information carriers for quantum information science and technologies. Obtaining valley polarization in TMDs, which is the equivalent of encoding or writing data, is the first step towards these applications. Here, we report valley polarization in monolayer TMD/chiral perovskite heterostructures at room temperature via spin selective charge transfer. We obtained 7% degree of polarization from the heterostructures after photoexcitation with a linearly polarized laser. We further demonstrate that charge transfer occurs within picoseconds using ultrafast transient absorption spectroscopy. Our results pave the way for practical valleytronics devices based on TMD/perovskite heterostructures.
2D transition metal dichalcogenides have two degenerate valleys which can be used to store and process information for quantum computing and communications. Here we report robust valley polarization in monolayer TMDs/chiral perovskite heterostructures at room temperature. We control valley index in monolayer TMDs via spin selective charge extraction with chiral perovskite and obtain 7% degree of polarization. We further investigate the charge transfer dynamics in the heterostructures by measuring transient absorption using pump-probe spectroscopy and show that charge transfer from monolayer TMD to chiral perovskite occurs within sub-picosecond timescales. Our results pave the way for practical valleytronics devices based on TMD/perovskite heterostructures.
Mixed dimensional heterostructures incorporating monolayers of transition metal dichalcogenides (TMDs) and non-van Der Waals nanomaterials provide interesting physics at low dimensions and beyond that of pristine TMDs. Of particular interest are light stimulated interfacial phenomena where excitons and/or charge carriers induced in either components diffuse at the interface of the heterostructure to provide enhanced optical activity.
In this work we report ultrafast carrier dynamics in mixed 0D/2D PbS QD-monolayer MoS2 by transient absorption microscopy and time-resolved confocal luminesce. We show dependency of the rate for carrier transfer with PbS QD core size which we relate to changes in bandgap alignment at interface and resulting from changes in components band overlap and provide a mechanistic view of interfacial charge transfer via ultrafast transient absorption and emission microscopy.
We report a nanohybrid based on an atomically thin, two-dimensional (2D) van Der Waals semiconductor, colloidal quantum dots and a light harvesting protein where step-wise energy transfer takes place. We connect nanocomponents of the nanohybrid via electrostatic self-assembly and layer-by-layer polyelectrolyte deposition and utilize bandgap engineering to created conditions for efficient directional stepwise energy transfer, from quantum dots, to proteins and to the 2D van Der Waals semiconductor, molybdenum diselenide (MoSe2). The biotic/abiotic nanohybrid exhibits enhanced absorption cross section and enhanced light harvesting through the addition of quantum dots and proteins next to MoSe2, which in turn leads to enhanced exciton generation in MoSe2 via energy transfer from quantum dots to proteins and finally to MoSe2.
High spatial and temporal resolution are key features for many modern applications, e.g. mass spectrometry, probing the structure of materials via neutron scattering, studying molecular structure, etc.1-5 Fast imaging also provides the capability of coincidence detection, and the further addition of sensitivity to single optical photons with the capability of timestamping them further broadens the field of potential applications. Photon counting is already widely used in X-ray imaging,6 where the high energy of the photons makes their detection easier.
TimepixCam is a novel optical imager,7 which achieves high spatial resolution using an array of 256×256 55 μm × 55μm pixels which have individually controlled functionality. It is based on a thin-entrance-window silicon sensor, bump-bonded to a Timepix ASIC.8 TimepixCam provides high quantum efficiency in the optical wavelength range (400-1000 nm).
We perform the timestamping of single photons with a time resolution of 20 ns, by coupling TimepixCam to a fast image-intensifier with a P47 phosphor screen. The fast emission time of the P479 allows us to preserve good time resolution while maintaining the capability to focus the optical output of the intensifier onto the 256×256 pixel Timepix sensor area. We demonstrate the capability of the (TimepixCam + image intensifier) setup to provide high-resolution single-photon timestamping, with an effective frame rate of 50 MHz.
Conducting nanoparticles with plasmon resonances create local, nanoscopic field enhancements that boost an analyte
molecule’s surface-averaged Raman scattering cross-section orders of magnitude above the bulk Raman cross-section by an amount known as the enhancement factor (EF). Demonstrations of single-molecule sensitivity with EF ~ 1013 have been reported from small “hot spots” (e.g., regions of enhanced electromagnetic near fields) on specialized substrates, but realistic chemical sensing requires high average EF over large substrates for practical sampling.1 By using simple wet chemical methods, NSRDEC scientists have fabricated large-area arrays of novel, highly conducting, anisotropic Ag and Al nanoparticles. The nanoparticles adhere to an ultrathin layer of poly-4(vinyl pyridine), and are anchored by submicron coating of poly-methyl methacrylate on glass and SiO2-coated Si substrates. The average interparticle spacing is determined by the dilution of the nanoparticle-water suspension. We present surface-enhanced Raman spectroscopy (SERS), spectrophotometry, and microscopy data from these nanoparticle arrays, model this data and the nanoscopic field enhancement, and determine the SERS EF. We compare the observed absorption resonances and SERS EF with those predicted by finite difference time domain modeling of the nanoscale fields and optical properties, and find good agreement between measured and calculated reflectivity, achieving EF ~ 106 for benzenethiol adsorbed onto a monolayer array of 120 nm Ag nanoparticles over an area of ~ 0.5 cm2. We discuss a way forward to increase SERS EF to 107 with large-area samples assembled using chemical methods, by using spiky Ag “nano-urchins” with very large predicted field enhancements.
Photo-induced electron transfer in CdSe/ZnS semiconductor quantum dot-fullerene conjugates was investigated by single
molecule fluorescence spectroscopy. The average rate for photoinduced electron transfer is estimated around 108s-1.
Quenching by electron transfer is observed in the "on" state and it manifests both as reduced fluorescence intensity and
as shortening in fluorescence lifetime. As a result, the electron transfer changes the on/off dynamics of the fluorescence
intensity of individual quantum dots.
We describe genetic engineering of a novel protein-nanoparticle hybrid system with great potential for patterning of
various types of nanoparticles and for biosensing applications. The hybrid system is based on a genetically-modified
chaperonin protein from the hyperthermophilic archaeon Sulfolobus shibatae. This chaperonin is an 18-subunit double
ring, which self-assembles in the presence of Mg ions and ATP. We describe a chaperonin mutant (His-β-
loopless:HBLL), with increased access to the central cavity and His-tags on each subunit extending into the central
cavity. This mutant binds water-soluble semiconductor quantum dots, creating a protein-encapsulated fluorescent
nanoparticle. By adding selective binding sites to the solvent-exposed regions of the chaperonin, this proteinnanoparticle
bioconjugate becomes a sensor for specific targets. Using a combination of biochemical and spectroscopic
assays, we characterize the formation, stoichiometry, affinity and stability of these novel sensors.
Proteins from Anthozoa species are homologous to the green fluorescent protein (GFP) from Aequorea victoria but with absorption/emission properties extended to longer wavelengths. HcRed is a far-red fluorescent protein originating from the sea anemone Heteractis crispa with absorption and emission maxima at 590 and 650 nm, respectively. We use ultrasensitive fluorescence spectroscopic methods to demonstrate that HcRed occurs as a dimer in solution and to explore the interaction between chromophores within such a dimer. We show that red chromophores within a dimer interact through a Förster-type fluorescence resonance energy transfer (FRET) mechanism. We present spectroscopic evidence for the presence of a yellow chromophore, an immature form of HcRed. This yellow chromophore is involved in directional FRET with the red chromophore when both types of chromophores are part of one dimer. We show that by combining ensemble and single molecule methods in the investigation of HcRed, we are able to sort out subpopulations of chromophores with different photophysical properties and to understand the mechanism of interaction between such chromophores. This study will help in future quantitative microscopy investigations that use HcRed as a fluorescent marker.
We use time-resolved single molecule fluorescence detection (MSMD) to investigate the fluorescence dynamics of a mutant of the wild-type Green Fluorescent Protein (GFP) from Aequorea victoria, the folding enhanced GFP (FEGFP). The folding enhanced GFP is a novel and robust variant designed for in vivo high-throughput screening of protein expression levels. This variant shows increased thermal stability and the ability to retain its fluorescence when fused to poorly folding proteins. Here we apply one- (OPE) and two- (TPE) photon excitation on freely diffusing FEGFP molecules. Under OPE, single FEGFP molecules undergo fluorescence flickering in the time scale of μs and tens of μs due to triplet formation and ground-state protonation-deprotonation, respectively. OPE fluorescence lifetimes of single FEGFP molecules show evidence for the presence of different emitting species, the I and B forms of FEGFP chromophore. TPE single FEGFP molecules flicker in fluorescence in the time scale of μs due to singlet-triplet transitions of the chromophore. Two-photon excitation of single FEGFP molecules results in the creation of a photoconverted species with a fluorescence lifetime of 2.5 ns, a species which is bright enough to be detected at the single molecule level. Our results indicate FEGFP is a promising fusion reporter for intracellular applications when
using OPE and TPE microscopy with single molecule sensitivity.
We report on the photophysical properties of a far-red intrinsic fluorescent protein by means of single molecule and ensemble spectroscopic methods. The green fluorescent protein (GFP) from Aequorea victoria is a popular fluorescent marker with genetically encoded fluorescence and which can be fused to any biological structure without affecting its function. GFP and its variants provide emission colors from blue to yellowish green. Red intrinsic fluorescent proteins from Anthozoa species represent a recent addition to the emission color palette provided by GFPs. Red intrinsic fluorescent markers are on high demand in protein-protein interaction studies based on fluorescence-resonance energy transfer or in multicolor tracking studies or in cellular investigations where autofluorescence possesses a problem. Here we address the photophysical properties of a far-red fluorescent protein (HcRed), a mutant engineered from a chromoprotein cloned from the sea anemone Heteractis crispa, by using a combination of ensemble and single molecule spectroscopic methods. We show evidence for the presence of HcRed protein as an oligomer and for incomplete maturation of its chromophore. Incomplete maturation results in the presence of an immature (yellow) species absorbing/fluorescing at 490/530-nm. This yellow chromophore is involved in a fast resonance-energy transfer with the mature (purple) chromophore. The mature chromophore of HcRed is found to adopt two conformations, a Transoriented form absorbing and 565-nm and non-fluorescent in solution and a Cis-oriented form absorbing at 590-nm and emitting at 645-nm. These two forms co-exist in solution in thermal equilibrium. Excitation-power dependence fluorescence correlation spectroscopy of HcRed shows evidence for singlet-triplet transitions in the microseconds time scale and for cis-trans isomerization occurring in a time scale of tens of microseconds. Single molecule fluorescence data recorded from immobilized HcRed proteins, all point to the presence of two classes of molecules: proteins with Cis and Trans-oriented chromophores. Immobilization of HcRed in water-filled pores of polyvinyl alcohol leads to a polymer matrix - protein barrel interaction which results in a 'freezing' of the chromophore in a stable conformation for which non-radiative deactivation pathways are either suppressed or reduced. As a result, proteins with both Cis- and Trans-oriented chromophores can be detected at the single molecule level. Polymer chain motion is suggested as a mediator for an eventual cis-trans isomerization of the chromophore in the case of single immobilized proteins.
Using single molecule fluorescence spectroscopy we have investigated fluorescence resonance energy transfer (FRET) occurring between two peryleneimide (PI) chromophores in a series of synthetic systems: PI end-capped fluorene trimers, hexamers and polymers for which the interchromophoric distance vary from 3.4 to 5.9 and 42 nm, respectively. By monitoring in parallel the fluorescence intensity and the number of independent emitting chromophores from each molecule, we could discriminate between competitive Foerster-type energy transfer processes such as energy hopping, singlet-singlet annihilation and singlet-triplet annihilation for the PI end-capped fluorine compounds. Due to different energy transfer efficiencies, variations in the interchromophoric distance enable switching between these processes. The single molecule fluorescence data reported here suggest that similar energy transfer pathways have to be considered in the analysis of single molecule trajectories of donor/acceptor pairs, as well as in the case of more complex systems like natural multichromophoric systems, such as light harvesting antennas or oligomeric fluorescent proteins.
One of the most intriguing findings in single molecule spectroscopy (SMS) is the observation of Raman spectra of
individual molecules, despite the small cross section of the transitions involved. The observation of the spectra can be
explained by the surface enhanced Raman scattering (SERRS) effect. At the single-molecule level, the SERRS-spectra
recorded as a function of time reveal inhomogeneous behaviour such as on/off blinking, spectral diffusion, intensity
fluctuations of vibrational line, and even splitting of some lines within the spectrum of one molecule. Single-molecule
SERRS (SM-SERRS) spectroscopy opens up exciting opportunities in the field of biophysics and biomedical
spectroscopy. The first example of single protein SERRS was performed on hemoglobin. However, the possibility of
extracting the heme group by silver sols can not be excluded. Here we report on SM-SERRS spectra of enhanced green
fluorescent protein (EGFP) in which the chromophore is kept in the protein. The time series of SM-SERRS spectra
suggest the conversion of the EGFP chromophore between the deprotonated and the protonated form. Autocorrelation
analysis of SM-SERRS trajectory reveals the presence of fast dynamics taking place in the protein. Our findings show
the potential of the technique to study structural dynamics of protein molecules.
We report on the fluorescence dynamics of a red fluorescent protein DsRed from the coral Discosoma genus by means of ensemble and single molecule fluorescence spectroscopy. Single molecule experiments performed on 543-nm excitation point to the existence of DsRed as a tetramer and reveal the presence of a no/off blinking phenomenon in the millisecond time range. Collective effects involving the red chromophores within the individual tetramers were observed. Time-resolved fluorescence data reveal the presence of a population of 25 % of the immature green chromophores which relates to tetramers containing only this immature green form and which is responsible for the weak fluorescence emitted by DsRed at 500-nm when excited at 460-nm. The remaining 75 % of the immature green chromophores are involved in a FRET process to the red chromophores within the tetramers that contain them. Using time-resolved detection and spectroscopy at single molecule level we were able to demonstrate the presence of a photoconversion process of the red chromophore emitting at 583-nm into a super red species that emits weakly at 595-nm. The same phenomenon is further corroborated at the ensemble level with the observation of the creation of a super red form and a blue absorbing species upon irradiation with 532-nm pulsed light at high excitation power.
Johan Hofkens, Tom Vosch, Mircea Cotlet, Satoshi Habuchi, Koen Van Der Biest, Klaus Mullen, Gunter Dirix, Jan Michiels, Jos Vanderleyden, Markus Sauer, Frans De Schryver
Multichromophoric systems play a key role in biological systems (light harvesting antenna complexes, fluorescent proteins...) and are equally important in material science applications (e.g. light emitting devices (LED) based on conjugated polymers). Our approach to get insight in the excited state processes of such systems is to make use of dendrimers labeled with photostable perylene dyes. Dendrimers synthesis indeed allows changing the number, relative position and orientation of attached chromophores in a controlled way. In the present contribution, excited state processes such as energy hopping, singlet-singlet annihilation, singlet-triplet annihilation are identified in individual tetrachromophoric dendrimers immobilized in a polymer matrix. Similar processes are then demonstrated to occur as well in immobilized tetramers of a red fluorescent protein from a coral of the Discosoma genus (DsRed).
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